diff --git a/src/core/include/mp-units/bits/quantity_cast.h b/src/core/include/mp-units/bits/quantity_cast.h
index 4fa7d08b..93954da2 100644
--- a/src/core/include/mp-units/bits/quantity_cast.h
+++ b/src/core/include/mp-units/bits/quantity_cast.h
@@ -51,9 +51,9 @@ template<QuantitySpec auto ToQS, typename Q>
 {
   if constexpr (detail::QuantityKindSpec<std::remove_const_t<decltype(ToQS)>> &&
                 AssociatedUnit<std::remove_const_t<decltype(Q::unit)>>)
-    return make_quantity<Q::unit>(std::forward<Q>(q).numerical_value_);
+    return make_quantity<Q::unit>(std::forward<Q>(q).numerical_value_is_an_implementation_detail_);
   else
-    return make_quantity<reference<ToQS, Q::unit>{}>(std::forward<Q>(q).numerical_value_);
+    return make_quantity<reference<ToQS, Q::unit>{}>(std::forward<Q>(q).numerical_value_is_an_implementation_detail_);
 }
 
 }  // namespace mp_units
diff --git a/src/core/include/mp-units/bits/sudo_cast.h b/src/core/include/mp-units/bits/sudo_cast.h
index 891b680f..bdc4f67e 100644
--- a/src/core/include/mp-units/bits/sudo_cast.h
+++ b/src/core/include/mp-units/bits/sudo_cast.h
@@ -63,10 +63,11 @@ template<Quantity To, typename From>
   if constexpr (q_unit == To::unit) {
     // no scaling of the number needed
     return make_quantity<To::reference>(static_cast<MP_UNITS_TYPENAME To::rep>(
-      std::forward<From>(q).numerical_value_));  // this is the only (and recommended) way to do
-                                                 // a truncating conversion on a number, so we are
-                                                 // using static_cast to suppress all the compiler
-                                                 // warnings on conversions
+      std::forward<From>(q)
+        .numerical_value_is_an_implementation_detail_));  // this is the only (and recommended) way to do
+                                                          // a truncating conversion on a number, so we are
+                                                          // using static_cast to suppress all the compiler
+                                                          // warnings on conversions
   } else {
     // scale the number
     constexpr Magnitude auto c_mag = get_canonical_unit(q_unit).mag / get_canonical_unit(To::unit).mag;
@@ -78,8 +79,9 @@ template<Quantity To, typename From>
     using multiplier_type =
       conditional<treat_as_floating_point<c_rep_type>, std::common_type_t<c_mag_type, long double>, c_mag_type>;
     constexpr auto val = [](Magnitude auto m) { return get_value<multiplier_type>(m); };
-    return static_cast<MP_UNITS_TYPENAME To::rep>(static_cast<c_rep_type>(std::forward<From>(q).numerical_value_) *
-                                                  val(num) / val(den) * val(irr)) *
+    return static_cast<MP_UNITS_TYPENAME To::rep>(
+             static_cast<c_rep_type>(std::forward<From>(q).numerical_value_is_an_implementation_detail_) * val(num) /
+             val(den) * val(irr)) *
            To::reference;
   }
 }
diff --git a/src/core/include/mp-units/quantity.h b/src/core/include/mp-units/quantity.h
index 2ebc1edd..3537fddb 100644
--- a/src/core/include/mp-units/quantity.h
+++ b/src/core/include/mp-units/quantity.h
@@ -85,7 +85,7 @@ using common_quantity_for = quantity<common_reference(Q1::reference, Q2::referen
 template<Reference auto R, RepresentationOf<get_quantity_spec(R).character> Rep = double>
 class quantity {
 public:
-  Rep numerical_value_;  // needs to be public for a structural type
+  Rep numerical_value_is_an_implementation_detail_;  // needs to be public for a structural type
 
   // member types and values
   static constexpr Reference auto reference = R;
@@ -126,7 +126,8 @@ public:
 
   template<detail::QuantityConvertibleTo<quantity> Q>
   constexpr explicit(!std::convertible_to<typename Q::rep, Rep>) quantity(const Q& q) :
-      numerical_value_(detail::sudo_cast<quantity>(q).numerical_value_)
+      numerical_value_is_an_implementation_detail_(
+        detail::sudo_cast<quantity>(q).numerical_value_is_an_implementation_detail_)
   {
   }
 
@@ -163,14 +164,14 @@ public:
     requires(U{} == unit)
   [[nodiscard]] constexpr rep& numerical_value_ref_in(U) & noexcept
   {
-    return numerical_value_;
+    return numerical_value_is_an_implementation_detail_;
   }
 
   template<Unit U>
     requires(U{} == unit)
   [[nodiscard]] constexpr const rep& numerical_value_ref_in(U) const& noexcept
   {
-    return numerical_value_;
+    return numerical_value_is_an_implementation_detail_;
   }
 
   template<Unit U>
@@ -181,39 +182,43 @@ public:
     requires requires(quantity q) { q.in(U{}); }
   [[nodiscard]] constexpr rep numerical_value_in(U) const noexcept
   {
-    return (*this).in(U{}).numerical_value_;
+    return (*this).in(U{}).numerical_value_is_an_implementation_detail_;
   }
 
   template<UnitCompatibleWith<unit, quantity_spec> U>
     requires requires(quantity q) { q.force_in(U{}); }
   [[nodiscard]] constexpr rep force_numerical_value_in(U) const noexcept
   {
-    return (*this).force_in(U{}).numerical_value_;
+    return (*this).force_in(U{}).numerical_value_is_an_implementation_detail_;
   }
 
   // conversion operators
   template<typename Q_, QuantityLike Q = std::remove_cvref_t<Q_>>
     requires detail::QuantityConvertibleTo<
       quantity, quantity<quantity_like_traits<Q>::reference, typename quantity_like_traits<Q>::rep>>
-  [[nodiscard]] explicit(is_specialization_of<decltype(quantity_like_traits<Q>::from_numerical_value(numerical_value_)),
+  [[nodiscard]] explicit(is_specialization_of<decltype(quantity_like_traits<Q>::from_numerical_value(
+                                                numerical_value_is_an_implementation_detail_)),
                                               convert_explicitly> ||
                          !std::convertible_to<Rep, typename Q::rep>) constexpr
-  operator Q_() const& noexcept(noexcept(quantity_like_traits<Q>::from_numerical_value(numerical_value_)) &&
-                                std::is_nothrow_copy_constructible_v<rep>)
+  operator Q_() const& noexcept(
+    noexcept(quantity_like_traits<Q>::from_numerical_value(numerical_value_is_an_implementation_detail_)) &&
+    std::is_nothrow_copy_constructible_v<rep>)
   {
-    return quantity_like_traits<Q>::from_numerical_value(numerical_value_).value;
+    return quantity_like_traits<Q>::from_numerical_value(numerical_value_is_an_implementation_detail_).value;
   }
 
   template<typename Q_, QuantityLike Q = std::remove_cvref_t<Q_>>
     requires detail::QuantityConvertibleTo<
       quantity, quantity<quantity_like_traits<Q>::reference, typename quantity_like_traits<Q>::rep>>
-  [[nodiscard]] explicit(is_specialization_of<decltype(quantity_like_traits<Q>::from_numerical_value(numerical_value_)),
+  [[nodiscard]] explicit(is_specialization_of<decltype(quantity_like_traits<Q>::from_numerical_value(
+                                                numerical_value_is_an_implementation_detail_)),
                                               convert_explicitly> ||
                          !std::convertible_to<Rep, typename Q::rep>) constexpr
-  operator Q_() && noexcept(noexcept(quantity_like_traits<Q>::from_numerical_value(numerical_value_)) &&
-                            std::is_nothrow_move_constructible_v<rep>)
+  operator Q_() && noexcept(
+    noexcept(quantity_like_traits<Q>::from_numerical_value(numerical_value_is_an_implementation_detail_)) &&
+    std::is_nothrow_move_constructible_v<rep>)
   {
-    return quantity_like_traits<Q>::from_numerical_value(std::move(numerical_value_)).value;
+    return quantity_like_traits<Q>::from_numerical_value(std::move(numerical_value_is_an_implementation_detail_)).value;
   }
 
   // member unary operators
@@ -224,7 +229,7 @@ public:
       } -> std::common_with<rep>;
     }
   {
-    return make_quantity<reference>(+numerical_value_);
+    return make_quantity<reference>(+numerical_value_is_an_implementation_detail_);
   }
 
   [[nodiscard]] constexpr Quantity auto operator-() const
@@ -234,7 +239,7 @@ public:
       } -> std::common_with<rep>;
     }
   {
-    return make_quantity<reference>(-numerical_value_);
+    return make_quantity<reference>(-numerical_value_is_an_implementation_detail_);
   }
 
   template<typename Q>
@@ -245,7 +250,7 @@ public:
       } -> std::same_as<rep&>;
     }
   {
-    ++q.numerical_value_;
+    ++q.numerical_value_is_an_implementation_detail_;
     return std::forward<Q>(q);
   }
 
@@ -256,7 +261,7 @@ public:
       } -> std::common_with<rep>;
     }
   {
-    return make_quantity<reference>(numerical_value_++);
+    return make_quantity<reference>(numerical_value_is_an_implementation_detail_++);
   }
 
   template<typename Q>
@@ -267,7 +272,7 @@ public:
       } -> std::same_as<rep&>;
     }
   {
-    --q.numerical_value_;
+    --q.numerical_value_is_an_implementation_detail_;
     return std::forward<Q>(q);
   }
 
@@ -278,7 +283,7 @@ public:
       } -> std::common_with<rep>;
     }
   {
-    return make_quantity<reference>(numerical_value_--);
+    return make_quantity<reference>(numerical_value_is_an_implementation_detail_--);
   }
 
   // compound assignment operators
@@ -290,7 +295,7 @@ public:
     }
   friend constexpr decltype(auto) operator+=(Q&& lhs, const quantity& rhs)
   {
-    lhs.numerical_value_ += rhs.numerical_value_;
+    lhs.numerical_value_is_an_implementation_detail_ += rhs.numerical_value_is_an_implementation_detail_;
     return std::forward<Q>(lhs);
   }
 
@@ -302,7 +307,7 @@ public:
     }
   friend constexpr decltype(auto) operator-=(Q&& lhs, const quantity& rhs)
   {
-    lhs.numerical_value_ -= rhs.numerical_value_;
+    lhs.numerical_value_is_an_implementation_detail_ -= rhs.numerical_value_is_an_implementation_detail_;
     return std::forward<Q>(lhs);
   }
 
@@ -317,7 +322,7 @@ public:
 
   {
     gsl_ExpectsAudit(rhs != zero());
-    lhs.numerical_value_ %= rhs.numerical_value_;
+    lhs.numerical_value_is_an_implementation_detail_ %= rhs.numerical_value_is_an_implementation_detail_;
     return std::forward<Q>(lhs);
   }
 
@@ -330,7 +335,7 @@ public:
              }
   friend constexpr decltype(auto) operator*=(Q&& lhs, const Value& v)
   {
-    lhs.numerical_value_ *= v;
+    lhs.numerical_value_is_an_implementation_detail_ *= v;
     return std::forward<Q>(lhs);
   }
 
@@ -343,7 +348,7 @@ public:
              }
   friend constexpr decltype(auto) operator*=(Q1&& lhs, const Q2& rhs)
   {
-    lhs.numerical_value_ *= rhs.numerical_value_;
+    lhs.numerical_value_is_an_implementation_detail_ *= rhs.numerical_value_is_an_implementation_detail_;
     return std::forward<Q1>(lhs);
   }
 
@@ -357,7 +362,7 @@ public:
   friend constexpr decltype(auto) operator/=(Q&& lhs, const Value& v)
   {
     gsl_ExpectsAudit(v != quantity_values<Value>::zero());
-    lhs.numerical_value_ /= v;
+    lhs.numerical_value_is_an_implementation_detail_ /= v;
     return std::forward<Q>(lhs);
   }
 
@@ -371,7 +376,7 @@ public:
   friend constexpr decltype(auto) operator/=(Q1&& lhs, const Q2& rhs)
   {
     gsl_ExpectsAudit(rhs != rhs.zero());
-    lhs.numerical_value_ /= rhs.numerical_value_;
+    lhs.numerical_value_is_an_implementation_detail_ /= rhs.numerical_value_is_an_implementation_detail_;
     return std::forward<Q1>(lhs);
   }
 
@@ -385,7 +390,7 @@ private:
 
   template<typename Value>
     requires std::constructible_from<rep, Value&&>
-  constexpr explicit quantity(Value&& v) : numerical_value_(std::forward<Value>(v))
+  constexpr explicit quantity(Value&& v) : numerical_value_is_an_implementation_detail_(std::forward<Value>(v))
   {
   }
 };
@@ -402,7 +407,10 @@ template<auto R1, typename Rep1, auto R2, typename Rep2>
 [[nodiscard]] constexpr Quantity auto operator+(const quantity<R1, Rep1>& lhs, const quantity<R2, Rep2>& rhs)
 {
   using ret = detail::common_quantity_for<std::plus<>, quantity<R1, Rep1>, quantity<R2, Rep2>>;
-  return make_quantity<ret::reference>(ret(lhs).numerical_value_ + ret(rhs).numerical_value_);
+  const ret ret_lhs(lhs);
+  const ret ret_rhs(rhs);
+  return make_quantity<ret::reference>(ret_lhs.numerical_value_ref_in(ret::unit) +
+                                       ret_rhs.numerical_value_ref_in(ret::unit));
 }
 
 template<auto R1, typename Rep1, auto R2, typename Rep2>
@@ -410,7 +418,10 @@ template<auto R1, typename Rep1, auto R2, typename Rep2>
 [[nodiscard]] constexpr Quantity auto operator-(const quantity<R1, Rep1>& lhs, const quantity<R2, Rep2>& rhs)
 {
   using ret = detail::common_quantity_for<std::minus<>, quantity<R1, Rep1>, quantity<R2, Rep2>>;
-  return make_quantity<ret::reference>(ret(lhs).numerical_value_ - ret(rhs).numerical_value_);
+  const ret ret_lhs(lhs);
+  const ret ret_rhs(rhs);
+  return make_quantity<ret::reference>(ret_lhs.numerical_value_ref_in(ret::unit) -
+                                       ret_rhs.numerical_value_ref_in(ret::unit));
 }
 
 template<auto R1, typename Rep1, auto R2, typename Rep2>
@@ -420,7 +431,10 @@ template<auto R1, typename Rep1, auto R2, typename Rep2>
 {
   gsl_ExpectsAudit(rhs != rhs.zero());
   using ret = detail::common_quantity_for<std::modulus<>, quantity<R1, Rep1>, quantity<R2, Rep2>>;
-  return make_quantity<ret::reference>(ret(lhs).numerical_value_ % ret(rhs).numerical_value_);
+  const ret ret_lhs(lhs);
+  const ret ret_rhs(rhs);
+  return make_quantity<ret::reference>(ret_lhs.numerical_value_ref_in(ret::unit) %
+                                       ret_rhs.numerical_value_ref_in(ret::unit));
 }
 
 template<auto R1, typename Rep1, auto R2, typename Rep2>
@@ -428,7 +442,8 @@ template<auto R1, typename Rep1, auto R2, typename Rep2>
                                   Rep2>
 [[nodiscard]] constexpr Quantity auto operator*(const quantity<R1, Rep1>& lhs, const quantity<R2, Rep2>& rhs)
 {
-  return make_quantity<R1 * R2>(lhs.numerical_value_ * rhs.numerical_value_);
+  return make_quantity<R1 * R2>(lhs.numerical_value_ref_in(quantity<R1, Rep1>::unit) *
+                                rhs.numerical_value_ref_in(quantity<R2, Rep2>::unit));
 }
 
 template<auto R, typename Rep, typename Value>
@@ -436,7 +451,7 @@ template<auto R, typename Rep, typename Value>
           detail::InvokeResultOf<get_quantity_spec(R).character, std::multiplies<>, Rep, const Value&>
 [[nodiscard]] constexpr Quantity auto operator*(const quantity<R, Rep>& q, const Value& v)
 {
-  return make_quantity<R>(q.numerical_value_ * v);
+  return make_quantity<R>(q.numerical_value_ref_in(quantity<R, Rep>::unit) * v);
 }
 
 template<typename Value, auto R, typename Rep>
@@ -444,7 +459,7 @@ template<typename Value, auto R, typename Rep>
           detail::InvokeResultOf<get_quantity_spec(R).character, std::multiplies<>, const Value&, Rep>
 [[nodiscard]] constexpr Quantity auto operator*(const Value& v, const quantity<R, Rep>& q)
 {
-  return make_quantity<R>(v * q.numerical_value_);
+  return make_quantity<R>(v * q.numerical_value_ref_in(quantity<R, Rep>::unit));
 }
 
 template<auto R1, typename Rep1, auto R2, typename Rep2>
@@ -452,7 +467,8 @@ template<auto R1, typename Rep1, auto R2, typename Rep2>
 [[nodiscard]] constexpr Quantity auto operator/(const quantity<R1, Rep1>& lhs, const quantity<R2, Rep2>& rhs)
 {
   gsl_ExpectsAudit(rhs != rhs.zero());
-  return make_quantity<R1 / R2>(lhs.numerical_value_ / rhs.numerical_value_);
+  return make_quantity<R1 / R2>(lhs.numerical_value_ref_in(quantity<R1, Rep1>::unit) /
+                                rhs.numerical_value_ref_in(quantity<R2, Rep2>::unit));
 }
 
 template<auto R, typename Rep, typename Value>
@@ -461,7 +477,7 @@ template<auto R, typename Rep, typename Value>
 [[nodiscard]] constexpr Quantity auto operator/(const quantity<R, Rep>& q, const Value& v)
 {
   gsl_ExpectsAudit(v != quantity_values<Value>::zero());
-  return make_quantity<R>(q.numerical_value_ / v);
+  return make_quantity<R>(q.numerical_value_ref_in(quantity<R, Rep>::unit) / v);
 }
 
 template<typename Value, auto R, typename Rep>
@@ -469,7 +485,7 @@ template<typename Value, auto R, typename Rep>
           detail::InvokeResultOf<get_quantity_spec(R).character, std::divides<>, const Value&, Rep>
 [[nodiscard]] constexpr Quantity auto operator/(const Value& v, const quantity<R, Rep>& q)
 {
-  return make_quantity<::mp_units::one / R>(v / q.numerical_value_);
+  return make_quantity<::mp_units::one / R>(v / q.numerical_value_ref_in(quantity<R, Rep>::unit));
 }
 
 template<auto R1, typename Rep1, auto R2, typename Rep2>
@@ -478,7 +494,9 @@ template<auto R1, typename Rep1, auto R2, typename Rep2>
 [[nodiscard]] constexpr bool operator==(const quantity<R1, Rep1>& lhs, const quantity<R2, Rep2>& rhs)
 {
   using ct = std::common_type_t<quantity<R1, Rep1>, quantity<R2, Rep2>>;
-  return ct(lhs).numerical_value_ == ct(rhs).numerical_value_;
+  const ct ct_lhs(lhs);
+  const ct ct_rhs(rhs);
+  return ct_lhs.numerical_value_ref_in(ct::unit) == ct_rhs.numerical_value_ref_in(ct::unit);
 }
 
 template<auto R1, typename Rep1, auto R2, typename Rep2>
@@ -487,7 +505,9 @@ template<auto R1, typename Rep1, auto R2, typename Rep2>
 [[nodiscard]] constexpr auto operator<=>(const quantity<R1, Rep1>& lhs, const quantity<R2, Rep2>& rhs)
 {
   using ct = std::common_type_t<quantity<R1, Rep1>, quantity<R2, Rep2>>;
-  return ct(lhs).numerical_value_ <=> ct(rhs).numerical_value_;
+  const ct ct_lhs(lhs);
+  const ct ct_rhs(rhs);
+  return ct_lhs.numerical_value_ref_in(ct::unit) <=> ct_rhs.numerical_value_ref_in(ct::unit);
 }
 
 // make_quantity
diff --git a/src/core/include/mp-units/quantity_spec.h b/src/core/include/mp-units/quantity_spec.h
index 37de04d9..900362dd 100644
--- a/src/core/include/mp-units/quantity_spec.h
+++ b/src/core/include/mp-units/quantity_spec.h
@@ -119,7 +119,8 @@ struct quantity_spec_interface {
     requires Quantity<std::remove_cvref_t<Q>> &&
              (explicitly_convertible(std::remove_reference_t<Q>::quantity_spec, self))
   {
-    return make_quantity<reference<self, std::remove_cvref_t<Q>::unit>{}>(std::forward<Q>(q).numerical_value_);
+    return make_quantity<reference<self, std::remove_cvref_t<Q>::unit>{}>(
+      std::forward<Q>(q).numerical_value_is_an_implementation_detail_);
   }
 #else
   template<typename Self_ = Self, UnitOf<Self_{}> U>
@@ -133,7 +134,8 @@ struct quantity_spec_interface {
              (explicitly_convertible(std::remove_reference_t<Q>::quantity_spec, Self_{}))
   [[nodiscard]] constexpr Quantity auto operator()(Q&& q) const
   {
-    return make_quantity<reference<Self{}, std::remove_cvref_t<Q>::unit>{}>(std::forward<Q>(q).numerical_value_);
+    return make_quantity<reference<Self{}, std::remove_cvref_t<Q>::unit>{}>(
+      std::forward<Q>(q).numerical_value_is_an_implementation_detail_);
   }
 #endif
 };
@@ -309,7 +311,8 @@ struct quantity_spec<Self, QS, Args...> : std::remove_const_t<decltype(QS)> {
              (explicitly_convertible(std::remove_reference_t<Q>::quantity_spec, Self_{}))
   [[nodiscard]] constexpr Quantity auto operator()(Q&& q) const
   {
-    return make_quantity<reference<Self{}, std::remove_cvref_t<Q>::unit>{}>(std::forward<Q>(q).numerical_value_);
+    return make_quantity<reference<Self{}, std::remove_cvref_t<Q>::unit>{}>(
+      std::forward<Q>(q).numerical_value_is_an_implementation_detail_);
   }
 #endif
 };
diff --git a/src/core/include/mp-units/reference.h b/src/core/include/mp-units/reference.h
index 41ac4787..f33e2b38 100644
--- a/src/core/include/mp-units/reference.h
+++ b/src/core/include/mp-units/reference.h
@@ -203,14 +203,16 @@ template<typename Q, Reference R>
   requires Quantity<std::remove_cvref_t<Q>>
 [[nodiscard]] constexpr Quantity auto operator*(Q&& q, R)
 {
-  return make_quantity<std::remove_cvref_t<Q>::reference * R{}>(std::forward<Q>(q).numerical_value_);
+  return make_quantity<std::remove_cvref_t<Q>::reference * R{}>(
+    std::forward<Q>(q).numerical_value_is_an_implementation_detail_);
 }
 
 template<typename Q, Reference R>
   requires Quantity<std::remove_cvref_t<Q>>
 [[nodiscard]] constexpr Quantity auto operator/(Q&& q, R)
 {
-  return make_quantity<std::remove_cvref_t<Q>::reference / R{}>(std::forward<Q>(q).numerical_value_);
+  return make_quantity<std::remove_cvref_t<Q>::reference / R{}>(
+    std::forward<Q>(q).numerical_value_is_an_implementation_detail_);
 }
 
 template<Reference R, typename Q>
diff --git a/test/unit_test/runtime/linear_algebra_test.cpp b/test/unit_test/runtime/linear_algebra_test.cpp
index 986204b9..492e3bf8 100644
--- a/test/unit_test/runtime/linear_algebra_test.cpp
+++ b/test/unit_test/runtime/linear_algebra_test.cpp
@@ -118,14 +118,14 @@ TEST_CASE("vector quantity", "[la]")
 
     SECTION("integral")
     {
-      SECTION("scalar on LHS") { CHECK((2 * v).numerical_value_ == vector<int>{2, 4, 6}); }
-      SECTION("scalar on RHS") { CHECK((v * 2).numerical_value_ == vector<int>{2, 4, 6}); }
+      SECTION("scalar on LHS") { CHECK((2 * v).numerical_value_in(m) == vector<int>{2, 4, 6}); }
+      SECTION("scalar on RHS") { CHECK((v * 2).numerical_value_in(m) == vector<int>{2, 4, 6}); }
     }
 
     SECTION("floating-point")
     {
-      SECTION("scalar on LHS") { CHECK((0.5 * v).numerical_value_ == vector<double>{0.5, 1., 1.5}); }
-      SECTION("scalar on RHS") { CHECK((v * 0.5).numerical_value_ == vector<double>{0.5, 1., 1.5}); }
+      SECTION("scalar on LHS") { CHECK((0.5 * v).numerical_value_in(m) == vector<double>{0.5, 1., 1.5}); }
+      SECTION("scalar on RHS") { CHECK((v * 0.5).numerical_value_in(m) == vector<double>{0.5, 1., 1.5}); }
     }
   }
 
@@ -133,8 +133,8 @@ TEST_CASE("vector quantity", "[la]")
   {
     const auto v = vector<int>{2, 4, 6} * isq::position_vector[m];
 
-    SECTION("integral") { CHECK((v / 2).numerical_value_ == vector<int>{1, 2, 3}); }
-    SECTION("floating-point") { CHECK((v / 0.5).numerical_value_ == vector<double>{4., 8., 12.}); }
+    SECTION("integral") { CHECK((v / 2).numerical_value_in(m) == vector<int>{1, 2, 3}); }
+    SECTION("floating-point") { CHECK((v / 0.5).numerical_value_in(m) == vector<double>{4., 8., 12.}); }
   }
 
   SECTION("add")
@@ -144,12 +144,12 @@ TEST_CASE("vector quantity", "[la]")
     SECTION("same unit")
     {
       const auto u = vector<int>{3, 2, 1} * isq::position_vector[m];
-      CHECK((v + u).numerical_value_ == vector<int>{4, 4, 4});
+      CHECK((v + u).numerical_value_in(m) == vector<int>{4, 4, 4});
     }
     SECTION("different units")
     {
       const auto u = vector<int>{3, 2, 1} * isq::position_vector[km];
-      CHECK((v + u).numerical_value_ == vector<int>{3001, 2002, 1003});
+      CHECK((v + u).numerical_value_in(m) == vector<int>{3001, 2002, 1003});
     }
   }
 
@@ -160,12 +160,12 @@ TEST_CASE("vector quantity", "[la]")
     SECTION("same unit")
     {
       const auto u = vector<int>{3, 2, 1} * isq::position_vector[m];
-      CHECK((v - u).numerical_value_ == vector<int>{-2, 0, 2});
+      CHECK((v - u).numerical_value_in(m) == vector<int>{-2, 0, 2});
     }
     SECTION("different units")
     {
       const auto u = vector<int>{3, 2, 1} * isq::position_vector[km];
-      CHECK((v - u).numerical_value_ == vector<int>{-2999, -1998, -997});
+      CHECK((v - u).numerical_value_in(m) == vector<int>{-2999, -1998, -997});
     }
   }
 
@@ -179,18 +179,18 @@ TEST_CASE("vector quantity", "[la]")
 
       SECTION("derived_quantity_spec")
       {
-        SECTION("scalar on LHS") { CHECK((mass * v).numerical_value_ == vector<int>{2, 4, 6}); }
-        SECTION("scalar on RHS") { CHECK((v * mass).numerical_value_ == vector<int>{2, 4, 6}); }
+        SECTION("scalar on LHS") { CHECK((mass * v).numerical_value_in(kg * m / s) == vector<int>{2, 4, 6}); }
+        SECTION("scalar on RHS") { CHECK((v * mass).numerical_value_in(kg * m / s) == vector<int>{2, 4, 6}); }
       }
       SECTION("quantity_cast to momentum")
       {
         SECTION("scalar on LHS")
         {
-          CHECK(quantity_cast<isq::momentum>(mass * v).numerical_value_ == vector<int>{2, 4, 6});
+          CHECK(quantity_cast<isq::momentum>(mass * v).numerical_value_in(N * s) == vector<int>{2, 4, 6});
         }
         SECTION("scalar on RHS")
         {
-          CHECK(quantity_cast<isq::momentum>(v * mass).numerical_value_ == vector<int>{2, 4, 6});
+          CHECK(quantity_cast<isq::momentum>(v * mass).numerical_value_in(N * s) == vector<int>{2, 4, 6});
         }
       }
       SECTION("quantity of momentum")
@@ -198,12 +198,12 @@ TEST_CASE("vector quantity", "[la]")
         SECTION("scalar on LHS")
         {
           const quantity<isq::momentum[N * s], vector<int>> momentum = mass * v;
-          CHECK(momentum.numerical_value_ == vector<int>{2, 4, 6});
+          CHECK(momentum.numerical_value_ref_in(N * s) == vector<int>{2, 4, 6});
         }
         SECTION("scalar on RHS")
         {
           const quantity<isq::momentum[N * s], vector<int>> momentum = v * mass;
-          CHECK(momentum.numerical_value_ == vector<int>{2, 4, 6});
+          CHECK(momentum.numerical_value_ref_in(N * s) == vector<int>{2, 4, 6});
         }
       }
     }
@@ -214,18 +214,18 @@ TEST_CASE("vector quantity", "[la]")
 
       SECTION("derived_quantity_spec")
       {
-        SECTION("scalar on LHS") { CHECK((mass * v).numerical_value_ == vector<double>{0.5, 1., 1.5}); }
-        SECTION("scalar on RHS") { CHECK((v * mass).numerical_value_ == vector<double>{0.5, 1., 1.5}); }
+        SECTION("scalar on LHS") { CHECK((mass * v).numerical_value_in(kg * m / s) == vector<double>{0.5, 1., 1.5}); }
+        SECTION("scalar on RHS") { CHECK((v * mass).numerical_value_in(kg * m / s) == vector<double>{0.5, 1., 1.5}); }
       }
       SECTION("quantity_cast to momentum")
       {
         SECTION("scalar on LHS")
         {
-          CHECK(quantity_cast<isq::momentum>(mass * v).numerical_value_ == vector<double>{0.5, 1., 1.5});
+          CHECK(quantity_cast<isq::momentum>(mass * v).numerical_value_in(N * s) == vector<double>{0.5, 1., 1.5});
         }
         SECTION("scalar on RHS")
         {
-          CHECK(quantity_cast<isq::momentum>(v * mass).numerical_value_ == vector<double>{0.5, 1., 1.5});
+          CHECK(quantity_cast<isq::momentum>(v * mass).numerical_value_in(N * s) == vector<double>{0.5, 1., 1.5});
         }
       }
       SECTION("quantity of momentum")
@@ -233,12 +233,12 @@ TEST_CASE("vector quantity", "[la]")
         SECTION("scalar on LHS")
         {
           const quantity<isq::momentum[N * s], vector<double>> momentum = mass * v;
-          CHECK(momentum.numerical_value_ == vector<double>{0.5, 1., 1.5});
+          CHECK(momentum.numerical_value_ref_in(N * s) == vector<double>{0.5, 1., 1.5});
         }
         SECTION("scalar on RHS")
         {
           const quantity<isq::momentum[N * s], vector<double>> momentum = v * mass;
-          CHECK(momentum.numerical_value_ == vector<double>{0.5, 1., 1.5});
+          CHECK(momentum.numerical_value_ref_in(N * s) == vector<double>{0.5, 1., 1.5});
         }
       }
     }
@@ -252,15 +252,15 @@ TEST_CASE("vector quantity", "[la]")
     {
       const auto dur = 2 * isq::duration[h];
 
-      SECTION("derived_quantity_spec") { CHECK((pos / dur).numerical_value_ == vector<int>{15, 10, 5}); }
+      SECTION("derived_quantity_spec") { CHECK((pos / dur).numerical_value_in(km / h) == vector<int>{15, 10, 5}); }
       SECTION("quantity_cast to velocity")
       {
-        CHECK(quantity_cast<isq::velocity>(pos / dur).numerical_value_ == vector<int>{15, 10, 5});
+        CHECK(quantity_cast<isq::velocity>(pos / dur).numerical_value_in(km / h) == vector<int>{15, 10, 5});
       }
       SECTION("quantity of velocity")
       {
         const quantity<isq::velocity[km / h], vector<int>> v = pos / dur;
-        CHECK(v.numerical_value_ == vector<int>{15, 10, 5});
+        CHECK(v.numerical_value_ref_in(km / h) == vector<int>{15, 10, 5});
       }
     }
 
@@ -268,15 +268,15 @@ TEST_CASE("vector quantity", "[la]")
     {
       const auto dur = 0.5 * isq::duration[h];
 
-      SECTION("derived_quantity_spec") { CHECK((pos / dur).numerical_value_ == vector<double>{60, 40, 20}); }
+      SECTION("derived_quantity_spec") { CHECK((pos / dur).numerical_value_in(km / h) == vector<double>{60, 40, 20}); }
       SECTION("quantity_cast to velocity")
       {
-        CHECK(quantity_cast<isq::velocity>(pos / dur).numerical_value_ == vector<double>{60, 40, 20});
+        CHECK(quantity_cast<isq::velocity>(pos / dur).numerical_value_in(km / h) == vector<double>{60, 40, 20});
       }
       SECTION("quantity of velocity")
       {
         const quantity<isq::velocity[km / h], vector<double>> v = pos / dur;
-        CHECK(v.numerical_value_ == vector<double>{60, 40, 20});
+        CHECK(v.numerical_value_ref_in(km / h) == vector<double>{60, 40, 20});
       }
     }
   }
diff --git a/test/unit_test/runtime/math_test.cpp b/test/unit_test/runtime/math_test.cpp
index fdf5e1dc..e468225f 100644
--- a/test/unit_test/runtime/math_test.cpp
+++ b/test/unit_test/runtime/math_test.cpp
@@ -90,12 +90,12 @@ TEST_CASE("numeric_limits functions", "[limits]")
 {
   SECTION("'epsilon' works as expected using default floating type")
   {
-    REQUIRE(epsilon<double>(isq::length[m]).numerical_value_ ==
+    REQUIRE(epsilon<double>(isq::length[m]).numerical_value_in(m) ==
             std::numeric_limits<decltype(1. * isq::length[m])::rep>::epsilon());
   }
   SECTION("'epsilon' works as expected using integers")
   {
-    REQUIRE(epsilon<int>(isq::length[m]).numerical_value_ ==
+    REQUIRE(epsilon<int>(isq::length[m]).numerical_value_in(m) ==
             std::numeric_limits<decltype(1 * isq::length[m])::rep>::epsilon());
   }
 }
diff --git a/test/unit_test/static/quantity_point_test.cpp b/test/unit_test/static/quantity_point_test.cpp
index 0368f30b..4674bfb2 100644
--- a/test/unit_test/static/quantity_point_test.cpp
+++ b/test/unit_test/static/quantity_point_test.cpp
@@ -240,14 +240,14 @@ static_assert(
 // static member functions
 ////////////////////////////
 
-static_assert(quantity_point<isq::height[m], mean_sea_level>::min().quantity_from_origin_.numerical_value_ ==
+static_assert(quantity_point<isq::height[m], mean_sea_level>::min().quantity_from_origin_.numerical_value_in(m) ==
               std::numeric_limits<double>::lowest());
-static_assert(quantity_point<isq::height[m], mean_sea_level>::max().quantity_from_origin_.numerical_value_ ==
+static_assert(quantity_point<isq::height[m], mean_sea_level>::max().quantity_from_origin_.numerical_value_in(m) ==
               std::numeric_limits<double>::max());
 
-static_assert(quantity_point<isq::height[m], ground_level, int>::min().quantity_from_origin_.numerical_value_ ==
+static_assert(quantity_point<isq::height[m], ground_level, int>::min().quantity_from_origin_.numerical_value_in(m) ==
               std::numeric_limits<int>::lowest());
-static_assert(quantity_point<isq::height[m], ground_level, int>::max().quantity_from_origin_.numerical_value_ ==
+static_assert(quantity_point<isq::height[m], ground_level, int>::max().quantity_from_origin_.numerical_value_in(m) ==
               std::numeric_limits<int>::max());
 
 
@@ -573,15 +573,15 @@ static_assert(is_of_type<(ground_level + isq::height(short(42) * m)).point_for(m
 // converting to a different unit
 ///////////////////////////////////
 
-static_assert((mean_sea_level + 2. * km).in(km).quantity_from_origin_.numerical_value_ == 2.);
-static_assert((mean_sea_level + 2. * km).in(m).quantity_from_origin_.numerical_value_ == 2000.);
-static_assert((mean_sea_level + 2000. * m).in(km).quantity_from_origin_.numerical_value_ == 2.);
-static_assert((ground_level + 2. * km).in(km).quantity_from_origin_.numerical_value_ == 2.);
-static_assert((ground_level + 2. * km).in(m).quantity_from_origin_.numerical_value_ == 2000.);
-static_assert((ground_level + 2000. * m).in(km).quantity_from_origin_.numerical_value_ == 2.);
-static_assert((tower_peak + 2. * km).in(km).quantity_from_origin_.numerical_value_ == 2.);
-static_assert((tower_peak + 2. * km).in(m).quantity_from_origin_.numerical_value_ == 2000.);
-static_assert((tower_peak + 2000. * m).in(km).quantity_from_origin_.numerical_value_ == 2.);
+static_assert((mean_sea_level + 2. * km).in(km).quantity_from_origin_.numerical_value_in(km) == 2.);
+static_assert((mean_sea_level + 2. * km).in(m).quantity_from_origin_.numerical_value_in(m) == 2000.);
+static_assert((mean_sea_level + 2000. * m).in(km).quantity_from_origin_.numerical_value_in(km) == 2.);
+static_assert((ground_level + 2. * km).in(km).quantity_from_origin_.numerical_value_in(km) == 2.);
+static_assert((ground_level + 2. * km).in(m).quantity_from_origin_.numerical_value_in(m) == 2000.);
+static_assert((ground_level + 2000. * m).in(km).quantity_from_origin_.numerical_value_in(km) == 2.);
+static_assert((tower_peak + 2. * km).in(km).quantity_from_origin_.numerical_value_in(km) == 2.);
+static_assert((tower_peak + 2. * km).in(m).quantity_from_origin_.numerical_value_in(m) == 2000.);
+static_assert((tower_peak + 2000. * m).in(km).quantity_from_origin_.numerical_value_in(km) == 2.);
 
 template<template<auto, auto, typename> typename QP>
 concept invalid_unit_conversion = requires {
@@ -652,14 +652,14 @@ static_assert([](auto v) {
 ////////////////////////
 
 // same type
-static_assert((mean_sea_level + 1 * m += 1 * m).quantity_from_origin_.numerical_value_ == 2);
-static_assert((mean_sea_level + 2 * m -= 1 * m).quantity_from_origin_.numerical_value_ == 1);
+static_assert((mean_sea_level + 1 * m += 1 * m).quantity_from_origin_.numerical_value_in(m) == 2);
+static_assert((mean_sea_level + 2 * m -= 1 * m).quantity_from_origin_.numerical_value_in(m) == 1);
 
 // different types
-static_assert((mean_sea_level + 2.5 * m += 3 * m).quantity_from_origin_.numerical_value_ == 5.5);
-static_assert((mean_sea_level + 123 * m += 1 * km).quantity_from_origin_.numerical_value_ == 1123);
-static_assert((mean_sea_level + 5.5 * m -= 3 * m).quantity_from_origin_.numerical_value_ == 2.5);
-static_assert((mean_sea_level + 1123 * m -= 1 * km).quantity_from_origin_.numerical_value_ == 123);
+static_assert((mean_sea_level + 2.5 * m += 3 * m).quantity_from_origin_.numerical_value_in(m) == 5.5);
+static_assert((mean_sea_level + 123 * m += 1 * km).quantity_from_origin_.numerical_value_in(m) == 1123);
+static_assert((mean_sea_level + 5.5 * m -= 3 * m).quantity_from_origin_.numerical_value_in(m) == 2.5);
+static_assert((mean_sea_level + 1123 * m -= 1 * km).quantity_from_origin_.numerical_value_in(m) == 123);
 
 
 template<template<auto, auto, typename> typename QP>
@@ -931,24 +931,24 @@ static_assert(is_of_type<(1 * m + tower_peak) - (1 * m + other_ground_level), qu
 
 // check for integral types promotion
 static_assert(is_same_v<decltype(((mean_sea_level + std::uint8_t(0) * m) + std::uint8_t(0) * m)
-                                   .quantity_from_origin_.numerical_value_),
+                                   .quantity_from_origin_.numerical_value_in(m)),
                         int>);
 static_assert(is_same_v<decltype((std::uint8_t(0) * m + (mean_sea_level + std::uint8_t(0) * m))
-                                   .quantity_from_origin_.numerical_value_),
+                                   .quantity_from_origin_.numerical_value_in(m)),
                         int>);
 static_assert(is_same_v<decltype(((mean_sea_level + std::uint8_t(0) * m) - std::uint8_t(0) * m)
-                                   .quantity_from_origin_.numerical_value_),
+                                   .quantity_from_origin_.numerical_value_in(m)),
+                        int>);
+static_assert(is_same_v<decltype(((mean_sea_level + std::uint8_t(0) * m) - (mean_sea_level + std::uint8_t(0) * m))
+                                   .numerical_value_in(m)),
                         int>);
-static_assert(
-  is_same_v<
-    decltype(((mean_sea_level + std::uint8_t(0) * m) - (mean_sea_level + std::uint8_t(0) * m)).numerical_value_), int>);
 static_assert(((mean_sea_level + std::uint8_t(128) * m) + std::uint8_t(128) * m)
-                .quantity_from_origin_.numerical_value_ == std::uint8_t(128) + std::uint8_t(128));
+                .quantity_from_origin_.numerical_value_in(m) == std::uint8_t(128) + std::uint8_t(128));
 static_assert((std::uint8_t(128) * m + (mean_sea_level + std::uint8_t(128) * m))
-                .quantity_from_origin_.numerical_value_ == std::uint8_t(128) + std::uint8_t(128));
-static_assert(((mean_sea_level + std::uint8_t(0) * m) - std::uint8_t(1) * m).quantity_from_origin_.numerical_value_ ==
-              std::uint8_t(0) - std::uint8_t(1));
-static_assert(((mean_sea_level + std::uint8_t(0) * m) - (mean_sea_level + std::uint8_t(1) * m)).numerical_value_ ==
+                .quantity_from_origin_.numerical_value_in(m) == std::uint8_t(128) + std::uint8_t(128));
+static_assert(((mean_sea_level + std::uint8_t(0) * m) - std::uint8_t(1) * m)
+                .quantity_from_origin_.numerical_value_in(m) == std::uint8_t(0) - std::uint8_t(1));
+static_assert(((mean_sea_level + std::uint8_t(0) * m) - (mean_sea_level + std::uint8_t(1) * m)).numerical_value_in(m) ==
               std::uint8_t(0) - std::uint8_t(1));
 
 // different representation types
@@ -1003,39 +1003,39 @@ static_assert(is_of_type<(mean_sea_level + 1 * km) - (mean_sea_level + 1. * m),
 static_assert(is_of_type<(mean_sea_level + 1. * km) - (mean_sea_level + 1. * m), quantity<si::metre, double>>);
 
 
-static_assert(((mean_sea_level + 1 * m) + 1 * m).quantity_from_origin_.numerical_value_ == 2);
-static_assert((1 * m + (mean_sea_level + 1 * m)).quantity_from_origin_.numerical_value_ == 2);
-static_assert(((mean_sea_level + 1 * m) + 1 * km).quantity_from_origin_.numerical_value_ == 1001);
-static_assert((1 * m + (mean_sea_level + 1 * km)).quantity_from_origin_.numerical_value_ == 1001);
-static_assert(((mean_sea_level + 1 * km) + 1 * m).quantity_from_origin_.numerical_value_ == 1001);
-static_assert((1 * km + (mean_sea_level + 1 * m)).quantity_from_origin_.numerical_value_ == 1001);
-static_assert(((mean_sea_level + 2 * m) - 1 * m).quantity_from_origin_.numerical_value_ == 1);
-static_assert(((mean_sea_level + 1 * km) - 1 * m).quantity_from_origin_.numerical_value_ == 999);
+static_assert(((mean_sea_level + 1 * m) + 1 * m).quantity_from_origin_.numerical_value_in(m) == 2);
+static_assert((1 * m + (mean_sea_level + 1 * m)).quantity_from_origin_.numerical_value_in(m) == 2);
+static_assert(((mean_sea_level + 1 * m) + 1 * km).quantity_from_origin_.numerical_value_in(m) == 1001);
+static_assert((1 * m + (mean_sea_level + 1 * km)).quantity_from_origin_.numerical_value_in(m) == 1001);
+static_assert(((mean_sea_level + 1 * km) + 1 * m).quantity_from_origin_.numerical_value_in(m) == 1001);
+static_assert((1 * km + (mean_sea_level + 1 * m)).quantity_from_origin_.numerical_value_in(m) == 1001);
+static_assert(((mean_sea_level + 2 * m) - 1 * m).quantity_from_origin_.numerical_value_in(m) == 1);
+static_assert(((mean_sea_level + 1 * km) - 1 * m).quantity_from_origin_.numerical_value_in(m) == 999);
 
-static_assert(((mean_sea_level + 1.5 * m) + 1 * m).quantity_from_origin_.numerical_value_ == 2.5);
-static_assert((1.5 * m + (mean_sea_level + 1 * m)).quantity_from_origin_.numerical_value_ == 2.5);
-static_assert(((mean_sea_level + 1.5 * m) + 1 * km).quantity_from_origin_.numerical_value_ == 1001.5);
-static_assert((1.5 * m + (mean_sea_level + 1 * km)).quantity_from_origin_.numerical_value_ == 1001.5);
-static_assert(((mean_sea_level + 1.5 * km) + 1 * m).quantity_from_origin_.numerical_value_ == 1501);
-static_assert((1.5 * km + (mean_sea_level + 1 * m)).quantity_from_origin_.numerical_value_ == 1501);
-static_assert(((mean_sea_level + 2.5 * m) - 1 * m).quantity_from_origin_.numerical_value_ == 1.5);
-static_assert(((mean_sea_level + 1.5 * km) - 1 * m).quantity_from_origin_.numerical_value_ == 1499);
+static_assert(((mean_sea_level + 1.5 * m) + 1 * m).quantity_from_origin_.numerical_value_in(m) == 2.5);
+static_assert((1.5 * m + (mean_sea_level + 1 * m)).quantity_from_origin_.numerical_value_in(m) == 2.5);
+static_assert(((mean_sea_level + 1.5 * m) + 1 * km).quantity_from_origin_.numerical_value_in(m) == 1001.5);
+static_assert((1.5 * m + (mean_sea_level + 1 * km)).quantity_from_origin_.numerical_value_in(m) == 1001.5);
+static_assert(((mean_sea_level + 1.5 * km) + 1 * m).quantity_from_origin_.numerical_value_in(m) == 1501);
+static_assert((1.5 * km + (mean_sea_level + 1 * m)).quantity_from_origin_.numerical_value_in(m) == 1501);
+static_assert(((mean_sea_level + 2.5 * m) - 1 * m).quantity_from_origin_.numerical_value_in(m) == 1.5);
+static_assert(((mean_sea_level + 1.5 * km) - 1 * m).quantity_from_origin_.numerical_value_in(m) == 1499);
 
-static_assert(((mean_sea_level + 1 * m) + 1.5 * m).quantity_from_origin_.numerical_value_ == 2.5);
-static_assert((1 * m + (mean_sea_level + 1.5 * m)).quantity_from_origin_.numerical_value_ == 2.5);
-static_assert(((mean_sea_level + 1 * m) + 1.5 * km).quantity_from_origin_.numerical_value_ == 1501);
-static_assert((1 * m + (mean_sea_level + 1.5 * km)).quantity_from_origin_.numerical_value_ == 1501);
-static_assert(((mean_sea_level + 1 * km) + 1.5 * m).quantity_from_origin_.numerical_value_ == 1001.5);
-static_assert((1 * km + (mean_sea_level + 1.5 * m)).quantity_from_origin_.numerical_value_ == 1001.5);
-static_assert(((mean_sea_level + 2 * m) - 1.5 * m).quantity_from_origin_.numerical_value_ == 0.5);
-static_assert(((mean_sea_level + 1 * km) - 1.5 * m).quantity_from_origin_.numerical_value_ == 998.5);
+static_assert(((mean_sea_level + 1 * m) + 1.5 * m).quantity_from_origin_.numerical_value_in(m) == 2.5);
+static_assert((1 * m + (mean_sea_level + 1.5 * m)).quantity_from_origin_.numerical_value_in(m) == 2.5);
+static_assert(((mean_sea_level + 1 * m) + 1.5 * km).quantity_from_origin_.numerical_value_in(m) == 1501);
+static_assert((1 * m + (mean_sea_level + 1.5 * km)).quantity_from_origin_.numerical_value_in(m) == 1501);
+static_assert(((mean_sea_level + 1 * km) + 1.5 * m).quantity_from_origin_.numerical_value_in(m) == 1001.5);
+static_assert((1 * km + (mean_sea_level + 1.5 * m)).quantity_from_origin_.numerical_value_in(m) == 1001.5);
+static_assert(((mean_sea_level + 2 * m) - 1.5 * m).quantity_from_origin_.numerical_value_in(m) == 0.5);
+static_assert(((mean_sea_level + 1 * km) - 1.5 * m).quantity_from_origin_.numerical_value_in(m) == 998.5);
 
-static_assert(((mean_sea_level + 2 * m) - (mean_sea_level + 1 * m)).numerical_value_ == 1);
-static_assert(((mean_sea_level + 1 * km) - (mean_sea_level + 1 * m)).numerical_value_ == 999);
-static_assert(((mean_sea_level + 2.5 * m) - (mean_sea_level + 1 * m)).numerical_value_ == 1.5);
-static_assert(((mean_sea_level + 1.5 * km) - (mean_sea_level + 1 * m)).numerical_value_ == 1499);
-static_assert(((mean_sea_level + 2 * m) - (mean_sea_level + 1.5 * m)).numerical_value_ == 0.5);
-static_assert(((mean_sea_level + 1 * km) - (mean_sea_level + 1.5 * m)).numerical_value_ == 998.5);
+static_assert(((mean_sea_level + 2 * m) - (mean_sea_level + 1 * m)).numerical_value_in(m) == 1);
+static_assert(((mean_sea_level + 1 * km) - (mean_sea_level + 1 * m)).numerical_value_in(m) == 999);
+static_assert(((mean_sea_level + 2.5 * m) - (mean_sea_level + 1 * m)).numerical_value_in(m) == 1.5);
+static_assert(((mean_sea_level + 1.5 * km) - (mean_sea_level + 1 * m)).numerical_value_in(m) == 1499);
+static_assert(((mean_sea_level + 2 * m) - (mean_sea_level + 1.5 * m)).numerical_value_in(m) == 0.5);
+static_assert(((mean_sea_level + 1 * km) - (mean_sea_level + 1.5 * m)).numerical_value_in(m) == 998.5);
 
 static_assert((mean_sea_level + 42 * m) - (ground_level + 42 * m) == -42 * m);
 static_assert((ground_level + 42 * m) - (mean_sea_level + 42 * m) == 42 * m);
diff --git a/test/unit_test/static/quantity_test.cpp b/test/unit_test/static/quantity_test.cpp
index 60c90c9e..47ed833c 100644
--- a/test/unit_test/static/quantity_test.cpp
+++ b/test/unit_test/static/quantity_test.cpp
@@ -108,14 +108,14 @@ static_assert(is_same_v<quantity<isq::length[m], int>::rep, int>);
 // static member functions
 ////////////////////////////
 
-static_assert(quantity<isq::length[m], int>::zero().numerical_value_ == 0);
-static_assert(quantity<isq::length[m], int>::one().numerical_value_ == 1);
-static_assert(quantity<isq::length[m], int>::min().numerical_value_ == std::numeric_limits<int>::lowest());
-static_assert(quantity<isq::length[m], int>::max().numerical_value_ == std::numeric_limits<int>::max());
-static_assert(quantity<isq::length[m], double>::zero().numerical_value_ == 0.0);
-static_assert(quantity<isq::length[m], double>::one().numerical_value_ == 1.0);
-static_assert(quantity<isq::length[m], double>::min().numerical_value_ == std::numeric_limits<double>::lowest());
-static_assert(quantity<isq::length[m], double>::max().numerical_value_ == std::numeric_limits<double>::max());
+static_assert(quantity<isq::length[m], int>::zero().numerical_value_in(m) == 0);
+static_assert(quantity<isq::length[m], int>::one().numerical_value_in(m) == 1);
+static_assert(quantity<isq::length[m], int>::min().numerical_value_in(m) == std::numeric_limits<int>::lowest());
+static_assert(quantity<isq::length[m], int>::max().numerical_value_in(m) == std::numeric_limits<int>::max());
+static_assert(quantity<isq::length[m], double>::zero().numerical_value_in(m) == 0.0);
+static_assert(quantity<isq::length[m], double>::one().numerical_value_in(m) == 1.0);
+static_assert(quantity<isq::length[m], double>::min().numerical_value_in(m) == std::numeric_limits<double>::lowest());
+static_assert(quantity<isq::length[m], double>::max().numerical_value_in(m) == std::numeric_limits<double>::max());
 
 
 //////////////////////////////
@@ -186,10 +186,10 @@ static_assert(std::convertible_to<quantity<isq::length[m], int>, quantity<isq::l
 // obtaining a number
 ///////////////////////
 
-static_assert(quantity<isq::length[m], int>(123 * m).numerical_value_ == 123);
-static_assert(quantity<isq::length[m], int>(2 * km).numerical_value_ == 2000);
-static_assert(quantity<isq::length[km], int>(2 * km).numerical_value_ == 2);
-static_assert(quantity<isq::length[km]>(1500 * m).numerical_value_ == 1.5);
+static_assert(quantity<isq::length[m], int>(123 * m).numerical_value_in(m) == 123);
+static_assert(quantity<isq::length[m], int>(2 * km).numerical_value_in(m) == 2000);
+static_assert(quantity<isq::length[km], int>(2 * km).numerical_value_in(km) == 2);
+static_assert(quantity<isq::length[km]>(1500 * m).numerical_value_in(km) == 1.5);
 
 
 ///////////////////////////////////
@@ -200,22 +200,22 @@ static_assert(is_of_type<(2. * km).in(m), quantity<si::metre>>);
 static_assert(is_of_type<isq::length(2. * km).in(m), quantity<isq::length[m]>>);
 static_assert(is_of_type<isq::height(2. * km).in(m), quantity<isq::height[m]>>);
 
-static_assert(quantity<isq::length[km]>(2. * km).in(km).numerical_value_ == 2.);
-static_assert(quantity<isq::length[km]>(2. * km).in(m).numerical_value_ == 2000.);
-static_assert(quantity<isq::length[m]>(2000. * m).in(km).numerical_value_ == 2.);
-static_assert(quantity<isq::length[km], int>(2 * km).in(km).numerical_value_ == 2);
-static_assert(quantity<isq::length[km], int>(2 * km).in(m).numerical_value_ == 2000);
+static_assert(quantity<isq::length[km]>(2. * km).in(km).numerical_value_in(km) == 2.);
+static_assert(quantity<isq::length[km]>(2. * km).in(m).numerical_value_in(m) == 2000.);
+static_assert(quantity<isq::length[m]>(2000. * m).in(km).numerical_value_in(km) == 2.);
+static_assert(quantity<isq::length[km], int>(2 * km).in(km).numerical_value_in(km) == 2);
+static_assert(quantity<isq::length[km], int>(2 * km).in(m).numerical_value_in(m) == 2000);
 
 static_assert(is_of_type<(2. * km).force_in(m), quantity<si::metre>>);
 static_assert(is_of_type<isq::length(2. * km).force_in(m), quantity<isq::length[m]>>);
 static_assert(is_of_type<isq::height(2. * km).force_in(m), quantity<isq::height[m]>>);
 
-static_assert(quantity<isq::length[km]>(2. * km).force_in(km).numerical_value_ == 2.);
-static_assert(quantity<isq::length[km]>(2. * km).force_in(m).numerical_value_ == 2000.);
-static_assert(quantity<isq::length[m]>(2000. * m).force_in(km).numerical_value_ == 2.);
-static_assert(quantity<isq::length[km], int>(2 * km).force_in(km).numerical_value_ == 2);
-static_assert(quantity<isq::length[km], int>(2 * km).force_in(m).numerical_value_ == 2000);
-static_assert(quantity<isq::length[m], int>(2000 * m).force_in(km).numerical_value_ == 2);
+static_assert(quantity<isq::length[km]>(2. * km).force_in(km).numerical_value_in(km) == 2.);
+static_assert(quantity<isq::length[km]>(2. * km).force_in(m).numerical_value_in(m) == 2000.);
+static_assert(quantity<isq::length[m]>(2000. * m).force_in(km).numerical_value_in(km) == 2.);
+static_assert(quantity<isq::length[km], int>(2 * km).force_in(km).numerical_value_in(km) == 2);
+static_assert(quantity<isq::length[km], int>(2 * km).force_in(m).numerical_value_in(m) == 2000);
+static_assert(quantity<isq::length[m], int>(2000 * m).force_in(km).numerical_value_in(km) == 2);
 
 template<template<auto, typename> typename Q>
 concept invalid_unit_conversion = requires {
@@ -317,28 +317,28 @@ static_assert([] {
   auto l1(1 * m), l2(2 * m);
   return l2 = l1;
 }()
-                .numerical_value_ == 1);
+                .numerical_value_in(m) == 1);
 static_assert([] {
   const auto l1(1 * m);
   auto l2(2 * m);
   return l2 = l1;
 }()
-                .numerical_value_ == 1);
+                .numerical_value_in(m) == 1);
 static_assert([]() {
   auto l1(1 * m), l2(2 * m);
   return l2 = std::move(l1);
 }()
-                .numerical_value_ == 1);
+                .numerical_value_in(m) == 1);
 
 
 ////////////////////
 // unary operators
 ////////////////////
 
-static_assert((+123 * m).numerical_value_ == 123);
-static_assert((-123 * m).numerical_value_ == -123);
-static_assert((+(-123 * m)).numerical_value_ == -123);
-static_assert((-(-123 * m)).numerical_value_ == 123);
+static_assert((+123 * m).numerical_value_in(m) == 123);
+static_assert((-123 * m).numerical_value_in(m) == -123);
+static_assert((+(-123 * m)).numerical_value_in(m) == -123);
+static_assert((-(-123 * m)).numerical_value_in(m) == 123);
 
 static_assert([](auto v) {
   auto vv = v++;
@@ -357,7 +357,7 @@ static_assert([](auto v) {
   return std::pair(v, vv);
 }(123 * m) == std::pair(122 * m, 122 * m));
 
-static_assert(is_same_v<decltype((+(short{0} * m)).numerical_value_), int>);
+static_assert(is_same_v<decltype((+(short{0} * m)).numerical_value_in(m)), int>);
 
 
 ////////////////////////
@@ -365,31 +365,31 @@ static_assert(is_same_v<decltype((+(short{0} * m)).numerical_value_), int>);
 ////////////////////////
 
 // same type
-static_assert((1 * m += 1 * m).numerical_value_ == 2);
-static_assert((2 * m -= 1 * m).numerical_value_ == 1);
-static_assert((1 * m *= 2).numerical_value_ == 2);
-static_assert((2 * m /= 2).numerical_value_ == 1);
-static_assert((1 * m *= 2 * one).numerical_value_ == 2);
-static_assert((2 * m /= 2 * one).numerical_value_ == 1);
-static_assert((7 * m %= 2 * m).numerical_value_ == 1);
+static_assert((1 * m += 1 * m).numerical_value_in(m) == 2);
+static_assert((2 * m -= 1 * m).numerical_value_in(m) == 1);
+static_assert((1 * m *= 2).numerical_value_in(m) == 2);
+static_assert((2 * m /= 2).numerical_value_in(m) == 1);
+static_assert((1 * m *= 2 * one).numerical_value_in(m) == 2);
+static_assert((2 * m /= 2 * one).numerical_value_in(m) == 1);
+static_assert((7 * m %= 2 * m).numerical_value_in(m) == 1);
 
 // different types
-static_assert((2.5 * m += 3 * m).numerical_value_ == 5.5);
-static_assert((123 * m += 1 * km).numerical_value_ == 1123);
-static_assert((5.5 * m -= 3 * m).numerical_value_ == 2.5);
-static_assert((1123 * m -= 1 * km).numerical_value_ == 123);
-static_assert((2.5 * m *= 3).numerical_value_ == 7.5);
-static_assert((7.5 * m /= 3).numerical_value_ == 2.5);
-static_assert((2.5 * m *= 3 * one).numerical_value_ == 7.5);
-static_assert((7.5 * m /= 3 * one).numerical_value_ == 2.5);
-static_assert((3500 * m %= 1 * km).numerical_value_ == 500);
+static_assert((2.5 * m += 3 * m).numerical_value_in(m) == 5.5);
+static_assert((123 * m += 1 * km).numerical_value_in(m) == 1123);
+static_assert((5.5 * m -= 3 * m).numerical_value_in(m) == 2.5);
+static_assert((1123 * m -= 1 * km).numerical_value_in(m) == 123);
+static_assert((2.5 * m *= 3).numerical_value_in(m) == 7.5);
+static_assert((7.5 * m /= 3).numerical_value_in(m) == 2.5);
+static_assert((2.5 * m *= 3 * one).numerical_value_in(m) == 7.5);
+static_assert((7.5 * m /= 3 * one).numerical_value_in(m) == 2.5);
+static_assert((3500 * m %= 1 * km).numerical_value_in(m) == 500);
 
-// static_assert((std::uint8_t(255) * m %= 256 * m).numerical_value_ == [] {
+// static_assert((std::uint8_t(255) * m %= 256 * m).numerical_value_in(m) == [] {
 //   std::uint8_t ui(255);
 //   return ui %= 256;
 // }());  // UB
 // TODO: Fix
-static_assert((std::uint8_t(255) * m %= 257 * m).numerical_value_ != [] {
+static_assert((std::uint8_t(255) * m %= 257 * m).numerical_value_in(m) != [] {
   std::uint8_t ui(255);
   return ui %= 257;
 }());
@@ -399,10 +399,10 @@ static_assert((std::uint8_t(255) * m %= 257 * m).numerical_value_ != [] {
 #ifndef MP_UNITS_COMP_MSVC
 // next two lines trigger conversions warnings
 // (warning disabled in CMake for this file)
-static_assert((22 * m *= 33.33).numerical_value_ == 733);
-static_assert((22 * m /= 3.33).numerical_value_ == 6);
-static_assert((22 * m *= 33.33 * one).numerical_value_ == 733);
-static_assert((22 * m /= 3.33 * one).numerical_value_ == 6);
+static_assert((22 * m *= 33.33).numerical_value_in(m) == 733);
+static_assert((22 * m /= 3.33).numerical_value_in(m) == 6);
+static_assert((22 * m *= 33.33 * one).numerical_value_in(m) == 733);
+static_assert((22 * m /= 3.33 * one).numerical_value_in(m) == 6);
 #endif
 
 template<template<auto, typename> typename Q>
@@ -530,9 +530,9 @@ static_assert(is_of_type<4 * one % (2 * one), quantity<one, int>>);
 // check for integral types promotion
 static_assert(is_same_v<decltype(std::uint8_t(0) * m + std::uint8_t(0) * m)::rep, int>);
 static_assert(is_same_v<decltype(std::uint8_t(0) * m - std::uint8_t(0) * m)::rep, int>);
-static_assert((std::uint8_t(128) * m + std::uint8_t(128) * m).numerical_value_ ==
+static_assert((std::uint8_t(128) * m + std::uint8_t(128) * m).numerical_value_in(m) ==
               std::uint8_t(128) + std::uint8_t(128));
-static_assert((std::uint8_t(0) * m - std::uint8_t(1) * m).numerical_value_ == std::uint8_t(0) - std::uint8_t(1));
+static_assert((std::uint8_t(0) * m - std::uint8_t(1) * m).numerical_value_in(m) == std::uint8_t(0) - std::uint8_t(1));
 
 static_assert(
   is_same_v<decltype((std::uint8_t(0) * m) % (std::uint8_t(0) * m))::rep, decltype(std::uint8_t(0) % std::uint8_t(0))>);
@@ -612,67 +612,67 @@ static_assert(is_of_type<1 * m / (1 * s), quantity<derived_unit<struct si::metre
 static_assert(is_of_type<1 * m / (1 * min), quantity<derived_unit<struct si::metre, per<struct si::minute>>{}, int>>);
 static_assert(is_of_type<1 * min / (1 * m), quantity<derived_unit<struct si::minute, per<struct si::metre>>{}, int>>);
 
-static_assert((1 * m + 1 * m).numerical_value_ == 2);
-static_assert((1 * m + 1 * km).numerical_value_ == 1001);
-static_assert((1 * km + 1 * m).numerical_value_ == 1001);
-static_assert((2 * m - 1 * m).numerical_value_ == 1);
-static_assert((1 * km - 1 * m).numerical_value_ == 999);
-static_assert((2 * m * 2).numerical_value_ == 4);
-static_assert((2 * m * (2 * one)).numerical_value_ == 4);
-static_assert((2 * m * (2 * percent)).numerical_value_ == 4);
-static_assert((3 * 3 * m).numerical_value_ == 9);
-static_assert(((3 * one) * (3 * m)).numerical_value_ == 9);
-static_assert(((3 * percent) * (3 * m)).numerical_value_ == 9);
-static_assert((4 * m / 2).numerical_value_ == 2);
-static_assert((4 * m / (2 * one)).numerical_value_ == 2);
-static_assert((4 * m / (2 * percent)).numerical_value_ == 2);
-static_assert((4 * km / (2 * m)).numerical_value_ == 2);
-static_assert((4000 * m / (2 * m)).numerical_value_ == 2000);
+static_assert((1 * m + 1 * m).numerical_value_in(m) == 2);
+static_assert((1 * m + 1 * km).numerical_value_in(m) == 1001);
+static_assert((1 * km + 1 * m).numerical_value_in(m) == 1001);
+static_assert((2 * m - 1 * m).numerical_value_in(m) == 1);
+static_assert((1 * km - 1 * m).numerical_value_in(m) == 999);
+static_assert((2 * m * 2).numerical_value_in(m) == 4);
+static_assert((2 * m * (2 * one)).numerical_value_in(m) == 4);
+static_assert((2 * m * (2 * percent)).numerical_value_in(percent * m) == 4);
+static_assert((3 * 3 * m).numerical_value_in(m) == 9);
+static_assert(((3 * one) * (3 * m)).numerical_value_in(m) == 9);
+static_assert(((3 * percent) * (3 * m)).numerical_value_in(percent * m) == 9);
+static_assert((4 * m / 2).numerical_value_in(m) == 2);
+static_assert((4 * m / (2 * one)).numerical_value_in(m) == 2);
+static_assert((4 * m / (2 * percent)).numerical_value_in(m / percent) == 2);
+static_assert((4 * km / (2 * m)).numerical_value_in(km / m) == 2);
+static_assert((4000 * m / (2 * m)).numerical_value_in(one) == 2000);
 
-static_assert((1.5 * m + 1 * m).numerical_value_ == 2.5);
-static_assert((1.5 * m + 1 * km).numerical_value_ == 1001.5);
-static_assert((1.5 * km + 1 * m).numerical_value_ == 1501);
-static_assert((2.5 * m - 1 * m).numerical_value_ == 1.5);
-static_assert((1.5 * km - 1 * m).numerical_value_ == 1499);
-static_assert((2.5 * m * 2).numerical_value_ == 5);
-static_assert((2.5 * m * (2 * one)).numerical_value_ == 5);
-static_assert((2.5 * m * (2 * percent)).numerical_value_ == 5);
-static_assert((2.5L * (2 * m)).numerical_value_ == 5);
-static_assert((2.5L * one * (2 * m)).numerical_value_ == 5);
-static_assert((2.5L * percent * (2 * m)).numerical_value_ == 5);
-static_assert((5. * m / 2).numerical_value_ == 2.5);
-static_assert((5. * m / (2 * one)).numerical_value_ == 2.5);
-static_assert((5. * m / (2 * percent)).numerical_value_ == 2.5);
-static_assert((5. * km / (2 * m)).numerical_value_ == 2.5);
-static_assert((5000. * m / (2 * m)).numerical_value_ == 2500);
+static_assert((1.5 * m + 1 * m).numerical_value_in(m) == 2.5);
+static_assert((1.5 * m + 1 * km).numerical_value_in(m) == 1001.5);
+static_assert((1.5 * km + 1 * m).numerical_value_in(m) == 1501);
+static_assert((2.5 * m - 1 * m).numerical_value_in(m) == 1.5);
+static_assert((1.5 * km - 1 * m).numerical_value_in(m) == 1499);
+static_assert((2.5 * m * 2).numerical_value_in(m) == 5);
+static_assert((2.5 * m * (2 * one)).numerical_value_in(m) == 5);
+static_assert((2.5 * m * (2 * percent)).numerical_value_in(m * percent) == 5);
+static_assert((2.5L * (2 * m)).numerical_value_in(m) == 5);
+static_assert((2.5L * one * (2 * m)).numerical_value_in(m) == 5);
+static_assert((2.5L * percent * (2 * m)).numerical_value_in(m * percent) == 5);
+static_assert((5. * m / 2).numerical_value_in(m) == 2.5);
+static_assert((5. * m / (2 * one)).numerical_value_in(m) == 2.5);
+static_assert((5. * m / (2 * percent)).numerical_value_in(m / percent) == 2.5);
+static_assert((5. * km / (2 * m)).numerical_value_in(km / m) == 2.5);
+static_assert((5000. * m / (2 * m)).numerical_value_in(one) == 2500);
 
-static_assert((1 * m + 1.5 * m).numerical_value_ == 2.5);
-static_assert((1 * m + 1.5 * km).numerical_value_ == 1501);
-static_assert((1 * km + 1.5 * m).numerical_value_ == 1001.5);
-static_assert((2 * m - 1.5 * m).numerical_value_ == 0.5);
-static_assert((1 * km - 1.5 * m).numerical_value_ == 998.5);
-static_assert((2 * m * 2.5L).numerical_value_ == 5);
-static_assert((2 * m * (2.5L * one)).numerical_value_ == 5);
-static_assert((2 * m * (2.5L * percent)).numerical_value_ == 5);
-static_assert((2 * 2.5 * m).numerical_value_ == 5);
-static_assert((2 * one * (2.5 * m)).numerical_value_ == 5);
-static_assert((2 * percent * (2.5 * m)).numerical_value_ == 5);
-static_assert((5 * m / 2.5L).numerical_value_ == 2);
-static_assert((5 * m / (2.5L * one)).numerical_value_ == 2);
-static_assert((5 * m / (2.5L * percent)).numerical_value_ == 2);
-static_assert((5 * km / (2.5 * m)).numerical_value_ == 2);
-static_assert((5000 * m / (2.5 * m)).numerical_value_ == 2000);
+static_assert((1 * m + 1.5 * m).numerical_value_in(m) == 2.5);
+static_assert((1 * m + 1.5 * km).numerical_value_in(m) == 1501);
+static_assert((1 * km + 1.5 * m).numerical_value_in(m) == 1001.5);
+static_assert((2 * m - 1.5 * m).numerical_value_in(m) == 0.5);
+static_assert((1 * km - 1.5 * m).numerical_value_in(m) == 998.5);
+static_assert((2 * m * 2.5L).numerical_value_in(m) == 5);
+static_assert((2 * m * (2.5L * one)).numerical_value_in(m) == 5);
+static_assert((2 * m * (2.5L * percent)).numerical_value_in(m * percent) == 5);
+static_assert((2 * 2.5 * m).numerical_value_in(m) == 5);
+static_assert((2 * one * (2.5 * m)).numerical_value_in(m) == 5);
+static_assert((2 * percent * (2.5 * m)).numerical_value_in(m * percent) == 5);
+static_assert((5 * m / 2.5L).numerical_value_in(m) == 2);
+static_assert((5 * m / (2.5L * one)).numerical_value_in(m) == 2);
+static_assert((5 * m / (2.5L * percent)).numerical_value_in(m / percent) == 2);
+static_assert((5 * km / (2.5 * m)).numerical_value_in(km / m) == 2);
+static_assert((5000 * m / (2.5 * m)).numerical_value_in(one) == 2000);
 
-static_assert((7 * m % (2 * m)).numerical_value_ == 1);
-static_assert((7 * km % (2000 * m)).numerical_value_ == 1000);
-static_assert((1300 * m % (1 * km)).numerical_value_ == 300);
-static_assert((7 * one % (2 * one)).numerical_value_ == 1);
+static_assert((7 * m % (2 * m)).numerical_value_in(m) == 1);
+static_assert((7 * km % (2000 * m)).numerical_value_in(m) == 1000);
+static_assert((1300 * m % (1 * km)).numerical_value_in(m) == 300);
+static_assert((7 * one % (2 * one)).numerical_value_in(one) == 1);
 
 static_assert((10 * m2 * (10 * m2)) / (50 * m2) == 2 * m2);
 
-static_assert((10 * km / (5 * m)).numerical_value_ == 2);
+static_assert((10 * km / (5 * m)).numerical_value_in(km / m) == 2);
 static_assert((10 * km / (5 * m)).numerical_value_in(one) == 2000);
-static_assert((10 * s * (2 * kHz)).numerical_value_ == 20);
+static_assert((10 * s * (2 * kHz)).numerical_value_in(s * kHz) == 20);
 
 // commutativity and associativity
 static_assert(10 * isq::length[si::metre] / (2 * isq::time[s]) + 5 * isq::speed[m / s] == 10 * isq::speed[m / s]);
@@ -758,9 +758,10 @@ static_assert(1 * one + 30 * percent == (100 + 30) * percent);
 
 static_assert(is_same_v<decltype(std::uint8_t(0) * one + std::uint8_t(0) * one)::rep, int>);
 static_assert(is_same_v<decltype(std::uint8_t(0) * one - std::uint8_t(0) * one)::rep, int>);
-static_assert((std::uint8_t(128) * one + std::uint8_t(128) * one).numerical_value_ ==
+static_assert((std::uint8_t(128) * one + std::uint8_t(128) * one).numerical_value_in(one) ==
               std::uint8_t(128) + std::uint8_t(128));
-static_assert((std::uint8_t(0) * one - std::uint8_t(1) * one).numerical_value_ == std::uint8_t(0) - std::uint8_t(1));
+static_assert((std::uint8_t(0) * one - std::uint8_t(1) * one).numerical_value_in(one) ==
+              std::uint8_t(0) - std::uint8_t(1));
 static_assert(is_same_v<decltype(std::uint8_t(0) * one % (std::uint8_t(0) * one))::rep,
                         decltype(std::uint8_t(0) % std::uint8_t(0))>);
 
@@ -905,14 +906,14 @@ static_assert((50. * percent).numerical_value_in(one) == 0.5);
 // value_cast
 //////////////////
 
-static_assert(value_cast<m>(2 * km).numerical_value_ == 2000);
-static_assert(value_cast<km>(2000 * m).numerical_value_ == 2);
-static_assert(value_cast<int>(1.23 * m).numerical_value_ == 1);
-static_assert(value_cast<km / h>(2000.0 * m / (3600.0 * s)).numerical_value_ == 2);
+static_assert(value_cast<m>(2 * km).numerical_value_in(m) == 2000);
+static_assert(value_cast<km>(2000 * m).numerical_value_in(km) == 2);
+static_assert(value_cast<int>(1.23 * m).numerical_value_in(m) == 1);
+static_assert(value_cast<km / h>(2000.0 * m / (3600.0 * s)).numerical_value_in(km / h) == 2);
 
-static_assert((2 * km).force_in(m).numerical_value_ == 2000);
-static_assert((2000 * m).force_in(km).numerical_value_ == 2);
-static_assert((2000.0 * m / (3600.0 * s)).force_in(km / h).numerical_value_ == 2);
+static_assert((2 * km).force_in(m).numerical_value_in(m) == 2000);
+static_assert((2000 * m).force_in(km).numerical_value_in(km) == 2);
+static_assert((2000.0 * m / (3600.0 * s)).force_in(km / h).numerical_value_in(km / h) == 2);
 
 //////////////////
 // quantity_cast