diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5dd89d06..f090756a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -43,4 +43,4 @@ enable_testing()
 add_subdirectory(test)
 
 # add usage example
-add_subdirectory(example)
+#add_subdirectory(example)
diff --git a/conanfile.py b/conanfile.py
index baa8d27b..44002a0f 100644
--- a/conanfile.py
+++ b/conanfile.py
@@ -6,13 +6,8 @@ class UnitConan(ConanFile):
     author = "Mateusz Pusz"
     license = "https://github.com/mpusz/units/blob/master/LICENSE"
     url = "https://github.com/mpusz/units"
-    description = "Physical Units library implementation for C++"
+    description = "Physical Units library for C++"
     settings = "os", "compiler", "build_type", "arch"
-    build_requires = (
-        "gtest/1.8.0@bincrafters/stable"
-    )
-    default_options = "gtest:shared=False"
-    generators = "cmake_paths"
 
     def build(self):
         cmake = CMake(self)
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 209d56af..629fa2a0 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -47,7 +47,7 @@ add_library(units INTERFACE)
 #    include/units/bits/quantity.h
 #    include/units/bits/tools.h
 #)
-target_compile_features(units INTERFACE cxx_std_17)
+target_compile_features(units INTERFACE cxx_std_20)
 target_include_directories(units
     INTERFACE
         $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
diff --git a/src/include/units/bits/stdconcepts.h b/src/include/units/bits/stdconcepts.h
new file mode 100644
index 00000000..5735dd24
--- /dev/null
+++ b/src/include/units/bits/stdconcepts.h
@@ -0,0 +1,44 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2018 Mateusz Pusz
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#pragma once
+
+#include <ratio>
+#include <type_traits>
+
+namespace mp {
+
+  namespace detail {
+    template<class T, class U>
+    bool concept SameHelper = std::is_same_v<T, U>;
+  }
+
+  template<class T, class U>
+  bool concept Same = detail::SameHelper<T, U> && detail::SameHelper<U, T>;
+
+  template <class From, class To>
+  bool concept ConvertibleTo = std::is_convertible_v<From, To> &&
+      requires(From (&f)()) {
+          static_cast<To>(f());
+      };
+
+}  // namespace mp
diff --git a/src/include/units/bits/tools.h b/src/include/units/bits/tools.h
index e4c4cd3b..9e74ee32 100644
--- a/src/include/units/bits/tools.h
+++ b/src/include/units/bits/tools.h
@@ -22,16 +22,12 @@
 
 #pragma once
 
+#include "stdconcepts.h"
 #include <ratio>
 #include <type_traits>
 
 namespace mp {
 
-  // Requires
-
-  template<bool B>
-  using Requires = std::enable_if_t<B, bool>;
-
   // static_sign
 
   template<std::intmax_t Pn>
@@ -60,6 +56,8 @@ namespace mp {
 
   // is_ratio
 
+  namespace detail {
+
   template<typename T>
   struct is_ratio : std::false_type {
   };
@@ -68,17 +66,22 @@ namespace mp {
   struct is_ratio<std::ratio<Num, Den>> : std::true_type {
   };
 
+  }
+
+  template<typename T>
+  bool concept Ratio = detail::is_ratio<T>::value;
+
   // common_ratio
 
   // todo: simplified
-  template<typename Ratio1, typename Ratio2>
+  template<Ratio Ratio1, Ratio Ratio2>
   struct common_ratio {
     using gcd_num = static_gcd<Ratio1::num, Ratio2::num>;
     using gcd_den = static_gcd<Ratio1::den, Ratio2::den>;
     using type = std::ratio<gcd_num::value, (Ratio1::den / gcd_den::value) * Ratio2::den>;
   };
 
-  template<typename Ratio1, typename Ratio2>
+  template<Ratio Ratio1, Ratio Ratio2>
   using common_ratio_t = typename common_ratio<Ratio1, Ratio2>::type;
 
 }  // namespace mp
diff --git a/src/include/units/bits/type_list.h b/src/include/units/bits/type_list.h
index 90c39081..da2bc7da 100644
--- a/src/include/units/bits/type_list.h
+++ b/src/include/units/bits/type_list.h
@@ -32,9 +32,22 @@ namespace mp {
   template<typename... Types>
   struct type_list;
 
+  namespace detail {
+
+    template<typename T>
+    struct is_type_list : std::false_type {};
+
+    template<typename... Types>
+    struct is_type_list<type_list<Types...>> : std::true_type {};
+
+  }
+
+  template<typename T>
+  bool concept TypeList = detail::is_type_list<T>::value;
+
   // push_front
 
-  template<typename TypeList, typename... Types>
+  template<TypeList List, typename... Types>
   struct type_list_push_front;
 
   template<typename... OldTypes, typename... NewTypes>
@@ -42,12 +55,12 @@ namespace mp {
     using type = type_list<NewTypes..., OldTypes...>;
   };
 
-  template<typename TypeList, typename... Types>
-  using type_list_push_front_t = typename type_list_push_front<TypeList, Types...>::type;
+  template<TypeList List, typename... Types>
+  using type_list_push_front_t = typename type_list_push_front<List, Types...>::type;
 
   // push_back
 
-  template<typename TypeList, typename... Types>
+  template<TypeList List, typename... Types>
   struct type_list_push_back;
 
   template<typename... OldTypes, typename... NewTypes>
@@ -55,8 +68,8 @@ namespace mp {
     using type = type_list<OldTypes..., NewTypes...>;
   };
 
-  template<typename TypeList, typename... Types>
-  using type_list_push_back_t = typename type_list_push_back<TypeList, Types...>::type;
+  template<TypeList List, typename... Types>
+  using type_list_push_back_t = typename type_list_push_back<List, Types...>::type;
 
   // split
 
@@ -76,17 +89,18 @@ namespace mp {
       using base = split_impl<Idx + 1, N, Rest...>;
       using base_first = typename base::first_list;
       using base_second = typename base::second_list;
-      using first_list = std::conditional_t < Idx<N, type_list_push_front_t<base_first, T>, base_first>;
-      using second_list = std::conditional_t < Idx<N, base_second, type_list_push_front_t<base_second, T>>;
+      using first_list = std::conditional_t<Idx<N, type_list_push_front_t<base_first, T>, base_first>;
+      using second_list = std::conditional_t<Idx<N, base_second, type_list_push_front_t<base_second, T>>;
     };
 
   }  // namespace detail
 
-  template<typename T, std::size_t N>
+  template<TypeList List, std::size_t N>
   struct type_list_split;
 
   template<std::size_t N, typename... Types>
   struct type_list_split<type_list<Types...>, N> {
+    static_assert(N <= sizeof...(Types), "Invalid index provided");
     using split = detail::split_impl<0, N, Types...>;
     using first_list = typename split::first_list;
     using second_list = typename split::second_list;
@@ -94,7 +108,7 @@ namespace mp {
 
   // split_half
 
-  template<typename T>
+  template<TypeList List>
   struct type_list_split_half;
 
   template<typename... Types>
@@ -103,11 +117,11 @@ namespace mp {
 
   // merge_sorted
 
-  template<typename T1, typename T2, template<typename, typename> typename Pred>
+  template<TypeList SortedList1, TypeList SortedList2, template<typename, typename> typename Pred>
   struct type_list_merge_sorted;
 
-  template<typename T1, typename T2, template<typename, typename> typename Pred>
-  using type_list_merge_sorted_t = typename type_list_merge_sorted<T1, T2, Pred>::type;
+  template<TypeList SortedList1, TypeList SortedList2, template<typename, typename> typename Pred>
+  using type_list_merge_sorted_t = typename type_list_merge_sorted<SortedList1, SortedList2, Pred>::type;
 
   template<typename... Lhs, template<typename, typename> typename Pred>
   struct type_list_merge_sorted<type_list<Lhs...>, type_list<>, Pred> {
@@ -132,7 +146,7 @@ namespace mp {
 
   // sort
 
-  template<typename T, template<typename, typename> typename Pred>
+  template<TypeList List, template<typename, typename> typename Pred>
   struct type_list_sort;
 
   template<template<typename, typename> typename Pred>
@@ -154,7 +168,7 @@ namespace mp {
     using type = type_list_merge_sorted_t<left, right, Pred>;
   };
 
-  template<typename T, template<typename, typename> typename Pred>
-  using type_list_sort_t = typename type_list_sort<T, Pred>::type;
+  template<TypeList TypeList, template<typename, typename> typename Pred>
+  using type_list_sort_t = typename type_list_sort<TypeList, Pred>::type;
 
 }  // namespace mp
\ No newline at end of file
diff --git a/src/include/units/dimension.h b/src/include/units/dimension.h
index e5c5dbba..092573d8 100644
--- a/src/include/units/dimension.h
+++ b/src/include/units/dimension.h
@@ -35,7 +35,7 @@ namespace units {
   // dim_id_less
 
   template<typename D1, typename D2>
-      struct dim_id_less : std::bool_constant<D1::value<D2::value> {
+      struct dim_id_less : std::bool_constant<D1::value < D2::value> {
   };
 
   // exp
@@ -46,15 +46,30 @@ namespace units {
     static constexpr int value = Value;
   };
 
+  // is_exp
+  namespace detail {
+    template<typename T>
+    struct is_exp : std::false_type {
+    };
+
+    template<typename BaseDim, int Value>
+    struct is_exp<exp<BaseDim, Value>> : std::true_type {
+    };
+  }
+
+  template<typename T>
+  bool concept Exp = detail::is_exp<T>::value;
+
+
   // exp_less
 
-  template<typename E1, typename E2>
+  template<Exp E1, Exp E2>
   struct exp_less : dim_id_less<typename E1::dimension, typename E2::dimension> {
   };
 
   // exp_invert
 
-  template<typename Exponent>
+  template<Exp Exponent>
   struct exp_invert;
 
   template<typename BaseDimension, int Value>
@@ -62,88 +77,96 @@ namespace units {
     using type = exp<BaseDimension, -Value>;
   };
 
-  template<typename Exponent>
+  template<Exp Exponent>
   using exp_invert_t = typename exp_invert<Exponent>::type;
 
   // dimension
 
-  template<typename... Exponents>
+  template<Exp... Exponents>
   using dimension = mp::type_list<Exponents...>;
 
+  // is_dimension
+  namespace detail {
+    template<typename T>
+    struct is_dimension : std::false_type {};
+
+    template<Exp... Exponents>
+    struct is_dimension<dimension<Exponents...>> : std::bool_constant<(is_exp<Exponents>::value && ...)> {};
+  }
+
+  template<typename T>
+  bool concept Dimension = detail::is_dimension<T>::value;
+
+
   // make_dimension
 
   namespace detail {
 
-    struct scalar;
-
-    template<typename D>
+    template<Dimension D>
     struct dim_consolidate;
 
-    template<typename D>
+    template<Dimension D>
     using dim_consolidate_t = typename dim_consolidate<D>::type;
 
     template<>
     struct dim_consolidate<dimension<>> {
-      using type = dimension<scalar>;
+      using type = dimension<>;
     };
 
-    template<typename E1>
-    struct dim_consolidate<dimension<E1>> {
-      using type = dimension<E1>;
+    template<Exp E>
+    struct dim_consolidate<dimension<E>> {
+      using type = dimension<E>;
     };
 
-    template<typename... ERest>
-    struct dim_consolidate<dimension<scalar, ERest...>> {
-      using type = dim_consolidate_t<dimension<ERest...>>;
+    template<Exp E1, Exp... ERest>
+    struct dim_consolidate<dimension<E1, ERest...>> {
+      using rest = dim_consolidate_t<dimension<ERest...>>;
+      using type = std::conditional_t<std::is_same_v<rest, dimension<>>, dimension<E1>,
+                                      mp::type_list_push_front_t<rest, E1>>;
     };
 
-    template<typename D, int V1, int V2, typename... ERest>
+    template<typename D, int V1, int V2, Exp... ERest>
     struct dim_consolidate<dimension<exp<D, V1>, exp<D, V2>, ERest...>> {
       using type = std::conditional_t<V1 + V2 == 0, dim_consolidate_t<dimension<ERest...>>,
                                       dim_consolidate_t<dimension<exp<D, V1 + V2>, ERest...>>>;
     };
 
-    template<typename E1, typename... ERest>
-    struct dim_consolidate<dimension<E1, ERest...>> {
-      using rest = dim_consolidate_t<dimension<ERest...>>;
-      using type = std::conditional_t<std::is_same_v<rest, dimension<scalar>>, dimension<E1>,
-                                      mp::type_list_push_front_t<rest, E1>>;
-    };
-
   }  // namespace detail
 
-  template<typename... Exponents>
+  template<Exp... Exponents>
   struct make_dimension {
-    using type = detail::dim_consolidate_t<mp::type_list_sort_t<mp::type_list<Exponents...>, exp_less>>;
+    using type = detail::dim_consolidate_t<mp::type_list_sort_t<dimension<Exponents...>, exp_less>>;
   };
 
-  template<typename... Exponents>
+  template<Exp... Exponents>
   using make_dimension_t = typename make_dimension<Exponents...>::type;
 
+
+
   // dimension_multiply
 
-  template<typename D1, typename D2>
+  template<Dimension D1, Dimension D2>
   struct dimension_multiply;
 
-  template<typename... Exp1, typename... Exp2>
+  template<Exp... Exp1, Exp... Exp2>
   struct dimension_multiply<dimension<Exp1...>, dimension<Exp2...>> {
     using type = make_dimension_t<Exp1..., Exp2...>;
   };
 
-  template<typename D1, typename D2>
+  template<Dimension D1, Dimension D2>
   using dimension_multiply_t = typename dimension_multiply<D1, D2>::type;
 
   // dimension_divide
 
-  template<typename D1, typename D2>
+  template<Dimension D1, Dimension D2>
   struct dimension_divide;
 
-  template<typename... Exp1, typename... Exp2>
+  template<Exp... Exp1, Exp... Exp2>
   struct dimension_divide<dimension<Exp1...>, dimension<Exp2...>>
       : dimension_multiply<dimension<Exp1...>, dimension<exp_invert_t<Exp2>...>> {
   };
 
-  template<typename D1, typename D2>
+  template<Dimension D1, Dimension D2>
   using dimension_divide_t = typename dimension_divide<D1, D2>::type;
 
 }  // namespace units
diff --git a/src/include/units/quantity.h b/src/include/units/quantity.h
index 54480c24..82123b0a 100644
--- a/src/include/units/quantity.h
+++ b/src/include/units/quantity.h
@@ -28,23 +28,26 @@
 
 namespace units {
 
-  template<typename Dimension, typename Rep, class Ratio>
-  class quantity;
-
   // is_quantity
 
+  template<Dimension Dimension, typename Rep, mp::Ratio Ratio>
+  class quantity;
+
   namespace detail {
 
     template<typename T>
     struct is_quantity : std::false_type {
     };
 
-    template<typename Dimension, typename Rep, class Ratio>
+    template<Dimension Dimension, typename Rep, mp::Ratio Ratio>
     struct is_quantity<quantity<Dimension, Rep, Ratio>> : std::true_type {
     };
 
   }
 
+  template<typename T>
+  bool concept Quantity = detail::is_quantity<T>::value;
+
   // treat_as_floating_point
 
   template<class Rep>
@@ -57,9 +60,9 @@ namespace units {
 
   namespace detail {
 
-    template<typename To, typename CRatio, typename CRep, bool NumIsOne = false, bool DenIsOne = false>
+    template<Quantity To, mp::Ratio CRatio, typename CRep, bool NumIsOne = false, bool DenIsOne = false>
     struct quantity_cast_impl {
-      template<typename Dimension, typename Rep, typename Ratio>
+      template<Dimension Dimension, typename Rep, mp::Ratio Ratio>
       static constexpr To cast(const quantity<Dimension, Rep, Ratio>& q)
       {
         return To(static_cast<typename To::rep>(static_cast<CRep>(q.count()) * static_cast<CRep>(CRatio::num) /
@@ -67,27 +70,27 @@ namespace units {
       }
     };
 
-    template<typename To, typename CRatio, typename CRep>
+    template<Quantity To, mp::Ratio CRatio, typename CRep>
     struct quantity_cast_impl<To, CRatio, CRep, true, true> {
-      template<typename Dimension, typename Rep, typename Ratio>
+      template<Dimension Dimension, typename Rep, mp::Ratio Ratio>
       static constexpr To cast(const quantity<Dimension, Rep, Ratio>& q)
       {
         return To(static_cast<typename To::rep>(q.count()));
       }
     };
 
-    template<typename To, typename CRatio, typename CRep>
+    template<Quantity To, mp::Ratio CRatio, typename CRep>
     struct quantity_cast_impl<To, CRatio, CRep, true, false> {
-      template<typename Dimension, typename Rep, typename Ratio>
+      template<Dimension Dimension, typename Rep, mp::Ratio Ratio>
       static constexpr To cast(const quantity<Dimension, Rep, Ratio>& q)
       {
         return To(static_cast<typename To::rep>(static_cast<CRep>(q.count()) / static_cast<CRep>(CRatio::den)));
       }
     };
 
-    template<typename To, typename CRatio, typename CRep>
+    template<Quantity To, mp::Ratio CRatio, typename CRep>
     struct quantity_cast_impl<To, CRatio, CRep, false, true> {
-      template<typename Dimension, typename Rep, typename Ratio>
+      template<Dimension Dimension, typename Rep, mp::Ratio Ratio>
       static constexpr To cast(const quantity<Dimension, Rep, Ratio>& q)
       {
         return To(static_cast<typename To::rep>(static_cast<CRep>(q.count()) * static_cast<CRep>(CRatio::num)));
@@ -96,8 +99,8 @@ namespace units {
 
   }  // namespace detail
 
-  template<typename To, typename Dimension, typename Rep, typename Ratio,
-           mp::Requires<detail::is_quantity<To>::value && std::is_same_v<typename To::dimension, Dimension>> = true>
+  template<Quantity To, Dimension Dimension, typename Rep, mp::Ratio Ratio>
+    requires mp::Same<typename To::dimension, Dimension>
   constexpr To quantity_cast(const quantity<Dimension, Rep, Ratio>& q)
   {
     using c_ratio = std::ratio_divide<Ratio, typename To::ratio>;
@@ -117,7 +120,7 @@ namespace units {
 
   // quantity
 
-  template<typename Dimension, typename Rep, class Ratio = std::ratio<1>>
+  template<Dimension Dimension, typename Rep, mp::Ratio Ratio = std::ratio<1>>
   class quantity {
     Rep value_;
   public:
@@ -126,21 +129,20 @@ namespace units {
     using ratio = Ratio;
 
     static_assert(!detail::is_quantity<Rep>::value, "rep cannot be a quantity");
-    static_assert(mp::is_ratio<ratio>::value, "ratio must be a specialization of std::ratio");
     static_assert(ratio::num > 0, "ratio must be positive");
 
     quantity() = default;
     quantity(const quantity&) = default;
 
-    template<class Rep2, mp::Requires<std::is_convertible_v<Rep2, rep> &&
-                                     (treat_as_floating_point_v<rep> || !treat_as_floating_point_v<Rep2>)> = true>
+    template<class Rep2>
+      requires mp::ConvertibleTo<Rep2, rep> && (treat_as_floating_point_v<rep> || !treat_as_floating_point_v<Rep2>)
     constexpr explicit quantity(const Rep2& r) : value_{static_cast<rep>(r)}
     {
     }
 
-    template<class Rep2, class Ratio2,
-             mp::Requires<treat_as_floating_point_v<rep> ||
-                         (std::ratio_divide<Ratio2, ratio>::den == 1 && !treat_as_floating_point<Rep2>::value)> = true>
+    template<class Rep2, mp::Ratio Ratio2>
+      requires mp::ConvertibleTo<Rep2, rep> && (treat_as_floating_point_v<rep> ||
+               (std::ratio_divide<Ratio2, ratio>::den == 1 && !treat_as_floating_point_v<Rep2>))
     constexpr quantity(const quantity<Dimension, Rep2, Ratio2>& q) : value_{quantity_cast<quantity>(q).count()}
     {
     }
@@ -208,7 +210,7 @@ namespace units {
   };
 
   // clang-format off
-  template<typename Dimension, typename Rep1, class Ratio1, typename Rep2, class Ratio2>
+  template<Dimension Dimension, typename Rep1, mp::Ratio Ratio1, typename Rep2, mp::Ratio Ratio2>
   std::common_type_t<quantity<Dimension, Rep1, Ratio1>, quantity<Dimension, Rep2, Ratio2>>
   constexpr operator+(const quantity<Dimension, Rep1, Ratio1>& lhs,
                       const quantity<Dimension, Rep2, Ratio2>& rhs)
@@ -217,7 +219,7 @@ namespace units {
     return ret(ret(lhs).count() + ret(rhs).count());
   }
 
-  template<typename Dimension, typename Rep1, class Ratio1, typename Rep2, class Ratio2>
+  template<Dimension Dimension, typename Rep1, mp::Ratio Ratio1, typename Rep2, mp::Ratio Ratio2>
   std::common_type_t<quantity<Dimension, Rep1, Ratio1>, quantity<Dimension, Rep2, Ratio2>>
   constexpr operator-(const quantity<Dimension, Rep1, Ratio1>& lhs,
                       const quantity<Dimension, Rep2, Ratio2>& rhs)
@@ -226,7 +228,7 @@ namespace units {
     return ret(ret(lhs).count() - ret(rhs).count());
   }
 
-  template<typename Dimension, typename Rep1, class Ratio, typename Rep2>
+  template<Dimension Dimension, typename Rep1, mp::Ratio Ratio, typename Rep2>
   quantity<Dimension, std::common_type_t<Rep1, Rep2>, Ratio>
   constexpr operator*(const quantity<Dimension, Rep1, Ratio>& q,
                       const Rep2& v)
@@ -235,7 +237,7 @@ namespace units {
     return ret(ret(q).count() * v);
   }
 
-  template<typename Dimension, typename Rep1, typename Rep2, class Ratio>
+  template<typename Rep1, Dimension Dimension, typename Rep2, mp::Ratio Ratio>
   quantity<Dimension, std::common_type_t<Rep1, Rep2>, Ratio>
   constexpr operator*(const Rep1& v,
                       const quantity<Dimension, Rep2, Ratio>& q)
@@ -243,8 +245,8 @@ namespace units {
     return q * v;
   }
 
-  template<typename Dimension1, typename Rep1, class Ratio1, typename Dimension2, typename Rep2, class Ratio2,
-      mp::Requires<treat_as_floating_point_v<std::common_type_t<Rep1, Rep2>> || std::ratio_multiply<Ratio1, Ratio2>::den == 1> = true>
+  template<Dimension Dimension1, typename Rep1, mp::Ratio Ratio1, Dimension Dimension2, typename Rep2, mp::Ratio Ratio2>
+      requires treat_as_floating_point_v<std::common_type_t<Rep1, Rep2>> || std::ratio_multiply<Ratio1, Ratio2>::den == 1
   quantity<dimension_multiply_t<Dimension1, Dimension2>, std::common_type_t<Rep1, Rep2>, std::ratio_multiply<Ratio1, Ratio2>>
   constexpr operator*(const quantity<Dimension1, Rep1, Ratio1>& lhs,
                       const quantity<Dimension2, Rep2, Ratio2>& rhs)
@@ -253,7 +255,7 @@ namespace units {
     return ret(lhs.count() * rhs.count());
   }
 
-  template<typename Rep1, typename Exponent, class Ratio, typename Rep2>
+  template<typename Rep1, Exp Exponent, mp::Ratio Ratio, typename Rep2>
   quantity<dimension<exp_invert_t<Exponent>>, std::common_type_t<Rep1, Rep2>, Ratio>
   constexpr operator/(const Rep1& v,
                       const quantity<dimension<Exponent>, Rep2, Ratio>& q)
@@ -263,7 +265,7 @@ namespace units {
     return ret(v / den(q).count());
   }
 
-  template<typename Dimension, typename Rep1, class Ratio, typename Rep2>
+  template<Dimension Dimension, typename Rep1, mp::Ratio Ratio, typename Rep2>
   quantity<Dimension, std::common_type_t<Rep1, Rep2>, Ratio>
   constexpr operator/(const quantity<Dimension, Rep1, Ratio>& q,
                       const Rep2& v)
@@ -272,7 +274,7 @@ namespace units {
     return ret(ret(q).count() / v);
   }
 
-  template<typename Dimension, typename Rep1, class Ratio1, typename Rep2, class Ratio2>
+  template<Dimension Dimension, typename Rep1, mp::Ratio Ratio1, typename Rep2, mp::Ratio Ratio2>
   std::common_type_t<Rep1, Rep2>
   constexpr operator/(const quantity<Dimension, Rep1, Ratio1>& lhs,
                       const quantity<Dimension, Rep2, Ratio2>& rhs)
@@ -281,8 +283,8 @@ namespace units {
     return cq(lhs).count() / cq(rhs).count();
   }
 
-  template<typename Dimension1, typename Rep1, class Ratio1, typename Dimension2, typename Rep2, class Ratio2,
-      mp::Requires<treat_as_floating_point_v<std::common_type_t<Rep1, Rep2>> || std::ratio_divide<Ratio1, Ratio2>::den == 1> = true>
+  template<Dimension Dimension1, typename Rep1, mp::Ratio Ratio1, Dimension Dimension2, typename Rep2, mp::Ratio Ratio2>
+      requires treat_as_floating_point_v<std::common_type_t<Rep1, Rep2>> || std::ratio_divide<Ratio1, Ratio2>::den == 1
   quantity<dimension_divide_t<Dimension1, Dimension2>, std::common_type_t<Rep1, Rep2>, std::ratio_divide<Ratio1, Ratio2>>
   constexpr operator/(const quantity<Dimension1, Rep1, Ratio1>& lhs,
                       const quantity<Dimension2, Rep2, Ratio2>& rhs)
@@ -291,7 +293,7 @@ namespace units {
     return ret(lhs.count() / rhs.count());
   }
 
-  template<typename Dimension, typename Rep1, class Ratio, typename Rep2>
+  template<Dimension Dimension, typename Rep1, mp::Ratio Ratio, typename Rep2>
   quantity<Dimension, std::common_type_t<Rep1, Rep2>, Ratio>
   constexpr operator%(const quantity<Dimension, Rep1, Ratio>& q,
                       const Rep2& v)
@@ -300,7 +302,7 @@ namespace units {
     return ret(ret(q).count() % v);
   }
 
-  template<typename Dimension, typename Rep1, class Ratio1, typename Rep2, class Ratio2>
+  template<Dimension Dimension, typename Rep1, mp::Ratio Ratio1, typename Rep2, mp::Ratio Ratio2>
   std::common_type_t<quantity<Dimension, Rep1, Ratio1>, quantity<Dimension, Rep2, Ratio2>>
   constexpr operator%(const quantity<Dimension, Rep1, Ratio1>& lhs,
                       const quantity<Dimension, Rep2, Ratio2>& rhs)
@@ -309,7 +311,9 @@ namespace units {
     return ret(ret(lhs).count() % ret(rhs).count());
   }
 
-  template<typename Dimension, typename Rep1, class Ratio1, typename Rep2, class Ratio2>
+  // clang-format on
+
+  template<Dimension Dimension, typename Rep1, mp::Ratio Ratio1, typename Rep2, mp::Ratio Ratio2>
   constexpr bool operator==(const quantity<Dimension, Rep1, Ratio1>& lhs,
                             const quantity<Dimension, Rep2, Ratio2>& rhs)
   {
@@ -317,14 +321,14 @@ namespace units {
     return ct(lhs).count() == ct(rhs).count();
   }
 
-  template<typename Dimension, typename Rep1, class Ratio1, typename Rep2, class Ratio2>
+  template<Dimension Dimension, typename Rep1, mp::Ratio Ratio1, typename Rep2, mp::Ratio Ratio2>
   constexpr bool operator!=(const quantity<Dimension, Rep1, Ratio1>& lhs,
                             const quantity<Dimension, Rep2, Ratio2>& rhs)
   {
     return !(lhs == rhs);
   }
 
-  template<typename Dimension, typename Rep1, class Ratio1, typename Rep2, class Ratio2>
+  template<Dimension Dimension, typename Rep1, mp::Ratio Ratio1, typename Rep2, mp::Ratio Ratio2>
   constexpr bool operator<(const quantity<Dimension, Rep1, Ratio1>& lhs,
                            const quantity<Dimension, Rep2, Ratio2>& rhs)
   {
@@ -332,34 +336,33 @@ namespace units {
     return ct(lhs).count() < ct(rhs).count();
   }
 
-  template<typename Dimension, typename Rep1, class Ratio1, typename Rep2, class Ratio2>
+  template<Dimension Dimension, typename Rep1, mp::Ratio Ratio1, typename Rep2, mp::Ratio Ratio2>
   constexpr bool operator<=(const quantity<Dimension, Rep1, Ratio1>& lhs,
                             const quantity<Dimension, Rep2, Ratio2>& rhs)
   {
     return !(rhs < lhs);
   }
 
-  template<typename Dimension, typename Rep1, class Ratio1, typename Rep2, class Ratio2>
+  template<Dimension Dimension, typename Rep1, mp::Ratio Ratio1, typename Rep2, mp::Ratio Ratio2>
   constexpr bool operator>(const quantity<Dimension, Rep1, Ratio1>& lhs,
                            const quantity<Dimension, Rep2, Ratio2>& rhs)
   {
     return rhs < lhs;
   }
 
-  template<typename Dimension, typename Rep1, class Ratio1, typename Rep2, class Ratio2>
+  template<Dimension Dimension, typename Rep1, mp::Ratio Ratio1, typename Rep2, mp::Ratio Ratio2>
   constexpr bool operator>=(const quantity<Dimension, Rep1, Ratio1>& lhs,
                             const quantity<Dimension, Rep2, Ratio2>& rhs)
   {
     return !(lhs < rhs);
   }
-  // clang-format on
 
 }  // namespace units
 
 namespace std {
 
   // todo: simplified
-  template<typename Dimension, typename Rep1, typename Ratio1, typename Rep2, typename Ratio2>
+  template<units::Dimension Dimension, typename Rep1, mp::Ratio Ratio1, typename Rep2, mp::Ratio Ratio2>
   struct common_type<units::quantity<Dimension, Rep1, Ratio1>, units::quantity<Dimension, Rep2, Ratio2>> {
     using type =
         units::quantity<Dimension, std::common_type_t<Rep1, Rep2>, mp::common_ratio_t<Ratio1, Ratio2>>;
diff --git a/src/include/units/si/base_dimensions.h b/src/include/units/si/base_dimensions.h
index 2a95d98d..6b22f5e5 100644
--- a/src/include/units/si/base_dimensions.h
+++ b/src/include/units/si/base_dimensions.h
@@ -23,7 +23,6 @@
 #pragma once
 
 #include "../dimension.h"
-#include "../quantity.h"
 
 namespace units {
 
diff --git a/src/include/units/si/length.h b/src/include/units/si/length.h
index 29e40049..58a0a892 100644
--- a/src/include/units/si/length.h
+++ b/src/include/units/si/length.h
@@ -23,6 +23,7 @@
 #pragma once
 
 #include "base_dimensions.h"
+#include "../quantity.h"
 
 namespace units {
 
diff --git a/src/include/units/si/time.h b/src/include/units/si/time.h
index db01847a..f1255623 100644
--- a/src/include/units/si/time.h
+++ b/src/include/units/si/time.h
@@ -23,6 +23,7 @@
 #pragma once
 
 #include "base_dimensions.h"
+#include "../quantity.h"
 
 namespace units {
 
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 4220865d..112dc8b5 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -37,16 +37,16 @@ if(NOT TARGET mp::units)
 endif()
 
 # unit tests
-add_executable(unit_tests
+add_library(unit_tests
     test_dimension.cpp
     test_quantity.cpp
+    test_tools.cpp
     test_type_list.cpp
     test_units.cpp
 )
 target_link_libraries(unit_tests
     PRIVATE
         mp::units
-        GTest::Main
 )
 add_test(NAME units.unit_tests
     COMMAND
diff --git a/test/test_dimension.cpp b/test/test_dimension.cpp
index 8607d264..dd09d602 100644
--- a/test/test_dimension.cpp
+++ b/test/test_dimension.cpp
@@ -21,7 +21,6 @@
 // SOFTWARE.
 
 #include "units/dimension.h"
-#include <gtest/gtest.h>
 #include <utility>
 
 using namespace units;
@@ -30,36 +29,44 @@ namespace {
 
   template<int Id, int Value>
   using e = exp<dim_id<Id>, Value>;
-}
 
-// make_dimension
+  // exp_invert
 
-static_assert(std::is_same_v<make_dimension_t<e<0, 1>>, dimension<e<0, 1>>>);
-static_assert(std::is_same_v<make_dimension_t<e<0, 1>, e<1, 1>>, dimension<e<0, 1>, e<1, 1>>>);
-static_assert(std::is_same_v<make_dimension_t<e<1, 1>, e<0, 1>>, dimension<e<0, 1>, e<1, 1>>>);
-static_assert(std::is_same_v<make_dimension_t<e<1, 1>, e<1, 1>>, dimension<e<1, 2>>>);
-static_assert(std::is_same_v<make_dimension_t<e<1, 1>, e<1, -1>>, dimension<detail::scalar>>);
+  static_assert(std::is_same_v<exp_invert_t<e<0, 1>>, e<0, -1>>);
+  static_assert(std::is_same_v<exp_invert_t<e<1, -1>>, e<1, 1>>);
 
-static_assert(std::is_same_v<make_dimension_t<e<0, 1>, e<1, 1>, e<0, 1>, e<1, 1>>, dimension<e<0, 2>, e<1, 2>>>);
-static_assert(std::is_same_v<make_dimension_t<e<0, -1>, e<1, -1>, e<0, -1>, e<1, -1>>, dimension<e<0, -2>, e<1, -2>>>);
+  // make_dimension
 
-static_assert(std::is_same_v<make_dimension_t<e<0, 1>, e<1, 1>, e<1, -1>>, dimension<e<0, 1>>>);
-static_assert(std::is_same_v<make_dimension_t<e<0, 1>, e<0, -1>, e<1, 1>>, dimension<e<1, 1>>>);
-static_assert(std::is_same_v<make_dimension_t<e<0, 1>, e<1, 1>, e<0, -1>>, dimension<e<1, 1>>>);
-static_assert(std::is_same_v<make_dimension_t<e<0, 1>, e<1, 1>, e<0, -1>, e<1, -1>>, dimension<detail::scalar>>);
+  static_assert(std::is_same_v<make_dimension_t<e<0, 1>>, dimension<e<0, 1>>>);
+  static_assert(std::is_same_v<make_dimension_t<e<0, 1>, e<1, 1>>, dimension<e<0, 1>, e<1, 1>>>);
+  static_assert(std::is_same_v<make_dimension_t<e<1, 1>, e<0, 1>>, dimension<e<0, 1>, e<1, 1>>>);
+  static_assert(std::is_same_v<make_dimension_t<e<1, 1>, e<1, 1>>, dimension<e<1, 2>>>);
+  static_assert(std::is_same_v<make_dimension_t<e<1, 1>, e<1, -1>>, dimension<>>);
 
-// dimension_multiply
+  static_assert(std::is_same_v<make_dimension_t<e<0, 1>, e<1, 1>, e<0, 1>, e<1, 1>>, dimension<e<0, 2>, e<1, 2>>>);
+  static_assert(
+      std::is_same_v<make_dimension_t<e<0, -1>, e<1, -1>, e<0, -1>, e<1, -1>>, dimension<e<0, -2>, e<1, -2>>>);
 
-static_assert(
-    std::is_same_v<dimension_multiply_t<dimension<e<0, 1>>, dimension<e<1, 1>>>, dimension<e<0, 1>, e<1, 1>>>);
-static_assert(std::is_same_v<dimension_multiply_t<dimension<e<0, 1>, e<1, 1>, e<2, 1>>, dimension<e<3, 1>>>,
-                             dimension<e<0, 1>, e<1, 1>, e<2, 1>, e<3, 1>>>);
-static_assert(std::is_same_v<dimension_multiply_t<dimension<e<0, 1>, e<1, 1>, e<2, 1>>, dimension<e<1, 1>>>,
-                             dimension<e<0, 1>, e<1, 2>, e<2, 1>>>);
-static_assert(std::is_same_v<dimension_multiply_t<dimension<e<0, 1>, e<1, 1>, e<2, 1>>, dimension<e<1, -1>>>,
-                             dimension<e<0, 1>, e<2, 1>>>);
+  static_assert(std::is_same_v<make_dimension_t<e<0, 1>, e<1, 1>, e<1, -1>>, dimension<e<0, 1>>>);
+  static_assert(std::is_same_v<make_dimension_t<e<0, 1>, e<0, -1>, e<1, 1>>, dimension<e<1, 1>>>);
+  static_assert(std::is_same_v<make_dimension_t<e<0, 1>, e<1, 1>, e<0, -1>>, dimension<e<1, 1>>>);
+  static_assert(std::is_same_v<make_dimension_t<e<0, 1>, e<1, 1>, e<0, -1>, e<1, -1>>, dimension<>>);
 
-// dimension_divide
+  // dimension_multiply
 
-static_assert(std::is_same_v<dimension_divide_t<dimension<e<0, 1>>, dimension<e<1, 1>>>, dimension<e<0, 1>, e<1, -1>>>);
-static_assert(std::is_same_v<dimension_divide_t<dimension<e<0, 1>>, dimension<e<0, 1>>>, dimension<detail::scalar>>);
+  static_assert(
+      std::is_same_v<dimension_multiply_t<dimension<e<0, 1>>, dimension<e<1, 1>>>, dimension<e<0, 1>, e<1, 1>>>);
+  static_assert(std::is_same_v<dimension_multiply_t<dimension<e<0, 1>, e<1, 1>, e<2, 1>>, dimension<e<3, 1>>>,
+                               dimension<e<0, 1>, e<1, 1>, e<2, 1>, e<3, 1>>>);
+  static_assert(std::is_same_v<dimension_multiply_t<dimension<e<0, 1>, e<1, 1>, e<2, 1>>, dimension<e<1, 1>>>,
+                               dimension<e<0, 1>, e<1, 2>, e<2, 1>>>);
+  static_assert(std::is_same_v<dimension_multiply_t<dimension<e<0, 1>, e<1, 1>, e<2, 1>>, dimension<e<1, -1>>>,
+                               dimension<e<0, 1>, e<2, 1>>>);
+
+  // dimension_divide
+
+  static_assert(
+      std::is_same_v<dimension_divide_t<dimension<e<0, 1>>, dimension<e<1, 1>>>, dimension<e<0, 1>, e<1, -1>>>);
+  static_assert(std::is_same_v<dimension_divide_t<dimension<e<0, 1>>, dimension<e<0, 1>>>, dimension<>>);
+
+}  // namespace
diff --git a/test/test_quantity.cpp b/test/test_quantity.cpp
index d4154d69..4793c141 100644
--- a/test/test_quantity.cpp
+++ b/test/test_quantity.cpp
@@ -21,153 +21,254 @@
 // SOFTWARE.
 
 #include "units/si/velocity.h"
-#include <gtest/gtest.h>
 #include <utility>
 
 using namespace units;
 using namespace units::literals;
 
+namespace {
 
+  template<typename T>
+  class my_value {
+    T data_{};
+
+  public:
+    my_value() = default;
+    constexpr my_value(T v) : data_{v} {}
+    constexpr operator T() const { return data_; }
+  };
+
+}  // namespace
+
+namespace units {
+
+  template<typename T>
+  struct treat_as_floating_point<my_value<T>> : std::is_floating_point<T> {
+  };
+
+  template<typename T>
+  struct quantity_values<my_value<T>> {
+    static constexpr my_value<T> zero() { return my_value<T>(0); }
+    static constexpr my_value<T> max() { return std::numeric_limits<T>::max(); }
+    static constexpr my_value<T> min() { return std::numeric_limits<T>::lowest(); }
+  };
+
+}  // namespace units
+
+namespace std {
+
+  template<typename T, typename U>
+  struct common_type<my_value<T>, U> : common_type<T, U> {
+  };
+
+  template<typename T, typename U>
+  struct common_type<T, my_value<U>> : common_type<T, U> {
+  };
+
+}  // namespace std
+
+namespace {
+
+  using namespace units;
+  using namespace units::literals;
+
+  // class invariants
+
+  //  constexpr quantity<meters<int>> error(0_m);  // should trigger a static_assert
+  //  constexpr quantity<int, float> error(0);  // should trigger a static_assert
+  //  constexpr quantity<int, std::ratio<-1, 1>> error(0);  // should trigger a static_assert
+
+  // member types
+
+  static_assert(std::is_same_v<meters<int>::rep, int>);
+  static_assert(std::is_same_v<meters<float>::rep, float>);
+  static_assert(std::is_same_v<meters<int>::ratio, std::ratio<1, 1>>);
+  static_assert(std::is_same_v<kilometers<int>::ratio, std::ratio<1000, 1>>);
+
+  // constructors
+
+  static_assert(meters<int>().count() == 0);
+  constexpr meters<int> kilometer{1000};
+  static_assert(kilometer.count() == 1000);
+  static_assert(meters<int>(kilometer).count() == kilometer.count());
+
+  static_assert(meters<int>(1).count() == 1);
+  static_assert(meters<int>(my_value<int>(1)).count() == 1);
+  static_assert(meters<my_value<int>>(1).count() == 1);
+  //  static_assert(meters<int>(1.0).count() == 1);   // should not compile
+  //  static_assert(meters<int>(my_value<float>(1.0)).count() == 1); // should not compile
+  //  static_assert(meters<my_value<int>>(1.0).count() == 1);   // should not compile
+  static_assert(meters<float>(1.0).count() == 1.0);
+  static_assert(meters<float>(my_value<float>(1.0)).count() == 1.0);
+  static_assert(meters<float>(1).count() == 1.0);
+  static_assert(meters<float>(my_value<int>(1)).count() == 1.0);
+  static_assert(meters<float>(3.14f).count() == 3.14f);
+  static_assert(meters<my_value<float>>(1.0).count() == 1.0);
+  static_assert(meters<my_value<float>>(1).count() == 1.0);
+  static_assert(meters<my_value<float>>(3.14f).count() == 3.14f);
+
+  static_assert(meters<int>(kilometer).count() == 1000);
+  //  static_assert(meters<int>(meters<float>(3.14)).count() == 3);   // should not compile
+  static_assert(meters<int>(quantity_cast<meters<int>>(3.14_m)).count() == 3);
+  //  static_assert(meters<int>(meters<my_value<float>>(1000.0)).count() == 1000);   // should not compile
+  //  static_assert(meters<my_value<int>>(1000.0_m).count() == 1000);   // should not compile
+  static_assert(meters<float>(1000.0_m).count() == 1000.0);
+  static_assert(meters<float>(meters<my_value<float>>(1000.0)).count() == 1000.0);
+  static_assert(meters<my_value<float>>(1000.0_m).count() == 1000.0);
+  static_assert(meters<float>(kilometer).count() == 1000.0);
+  static_assert(meters<my_value<float>>(kilometer).count() == 1000.0);
+  static_assert(meters<int>(1_km).count() == 1000);
+  //  static_assert(meters<int>(1_s).count() == 1);   // should not compile
+  //  static_assert(kilometers<int>(1010_m).count() == 1);   // should not compile
+  static_assert(kilometers<int>(quantity_cast<kilometers<int>>(1010_m)).count() == 1);
+
+  // assignment operator
+
+  static_assert([]() {
+    meters<int> l1(1), l2(2);
+    return l2 = l1;
+  }()
+                    .count() == 1);
+
+  // static member functions
+
+  static_assert(meters<int>::zero().count() == 0);
+  static_assert(meters<int>::min().count() == std::numeric_limits<int>::lowest());
+  static_assert(meters<int>::max().count() == std::numeric_limits<int>::max());
+  static_assert(meters<float>::zero().count() == 0.0);
+  static_assert(meters<float>::min().count() == std::numeric_limits<float>::lowest());
+  static_assert(meters<float>::max().count() == std::numeric_limits<float>::max());
+  static_assert(meters<my_value<int>>::zero().count() == 0);
+  static_assert(meters<my_value<int>>::min().count() == std::numeric_limits<int>::lowest());
+  static_assert(meters<my_value<int>>::max().count() == std::numeric_limits<int>::max());
+  static_assert(meters<my_value<float>>::zero().count() == 0.0);
+  static_assert(meters<my_value<float>>::min().count() == std::numeric_limits<float>::lowest());
+  static_assert(meters<my_value<float>>::max().count() == std::numeric_limits<float>::max());
+
+  // unary member operators
+
+  static_assert((+kilometer).count() == 1000);
+  static_assert((-kilometer).count() == -1000);
+  static_assert((+(-kilometer)).count() == -1000);
+  static_assert((-(-kilometer)).count() == 1000);
+
+  // binary member operators
+
+  static_assert([](auto v) {
+    auto vv = v++;
+    return std::make_pair(v, vv);
+  }(kilometer) == std::make_pair(meters<int>(1001), meters<int>(1000)));
+  static_assert([](auto v) {
+    auto vv = ++v;
+    return std::make_pair(v, vv);
+  }(kilometer) == std::make_pair(meters<int>(1001), meters<int>(1001)));
+  static_assert([](auto v) {
+    auto vv = v--;
+    return std::make_pair(v, vv);
+  }(kilometer) == std::make_pair(meters<int>(999), meters<int>(1000)));
+  static_assert([](auto v) {
+    auto vv = --v;
+    return std::make_pair(v, vv);
+  }(kilometer) == std::make_pair(meters<int>(999), meters<int>(999)));
+
+  // compound assignment
+
+  static_assert((1_m += 1_m).count() == 2);
+  static_assert((2_m -= 1_m).count() == 1);
+  static_assert((1_m *= 2).count() == 2);
+  static_assert((2_m /= 2).count() == 1);
+  static_assert((7_m %= 2).count() == 1);
+  static_assert((7_m %= 2_m).count() == 1);
+
+  // non-member arithmetic operators
+
+  static_assert(std::is_same_v<decltype(meters<int>() + meters<float>()), meters<float>>);
+  static_assert(std::is_same_v<decltype(meters<float>() - meters<int>()), meters<float>>);
+  static_assert(std::is_same_v<decltype(meters<int>() * 1.0f), meters<float>>);
+  static_assert(std::is_same_v<decltype(1.0f * meters<int>()), meters<float>>);
+  static_assert(std::is_same_v<decltype(meters<int>() / 1.0f), meters<float>>);
+  static_assert(std::is_same_v<decltype(meters<int>() / meters<float>()), float>);
+  static_assert(std::is_same_v<decltype(meters<int>() % short(1)), meters<int>>);
+  static_assert(std::is_same_v<decltype(meters<int>() % meters<short>(1)), meters<int>>);
+
+  static_assert((1_m + kilometer).count() == 1001);
+  static_assert((1_m + 1_km).count() == 1001);
+  static_assert((kilometer - 1_m).count() == 999);
+  static_assert((1_km - 1_m).count() == 999);
+  static_assert((2_m * 2).count() == 4);
+  static_assert((3 * 3_m).count() == 9);
+  static_assert((4_m / 2).count() == 2);
+  static_assert(4_m / 2_m == 2);
+  static_assert(4_km / 2000_m == 2);
+  static_assert((7_m % 2).count() == 1);
+  static_assert((7_m % 2_m).count() == 1);
+  static_assert((7_km % 2000_m).count() == 1000);
+
+  // comparators
+
+  static_assert(2_m + 1_m == 3_m);
+  static_assert(!(2_m + 2_m == 3_m));
+  static_assert(2_m + 2_m != 3_m);
+  static_assert(!(2_m + 2_m != 4_m));
+  static_assert(2_m > 1_m);
+  static_assert(!(1_m > 1_m));
+  static_assert(1_m < 2_m);
+  static_assert(!(2_m < 2_m));
+  static_assert(2_m >= 1_m);
+  static_assert(2_m >= 2_m);
+  static_assert(!(2_m >= 3_m));
+  static_assert(1_m <= 2_m);
+  static_assert(2_m <= 2_m);
+  static_assert(!(3_m <= 2_m));
+
+  static_assert(3_m == 3.0_m);
+  static_assert(3_m != 3.14_m);
+  static_assert(2_m > 1.0_m);
+  static_assert(1.0_m < 2_m);
+  static_assert(2.0_m >= 1_m);
+  static_assert(1_m <= 2.0_m);
+
+  static_assert(1000_m == 1_km);
+  static_assert(1001_m != 1_km);
+  static_assert(1001_m > 1_km);
+  static_assert(999_m < 1_km);
+  static_assert(1000_m >= 1_km);
+  static_assert(1000_m <= 1_km);
 
 // is_quantity
 
-static_assert(units::detail::is_quantity<millimeters<int>>::value);
+  static_assert(units::detail::is_quantity<millimeters<int>>::value);
 
+  // common_type
 
-// common_type
+  static_assert(std::is_same_v<std::common_type_t<meters<int>, kilometers<int>>, meters<int>>);
+  static_assert(std::is_same_v<std::common_type_t<kilometers<long long>, meters<int>>, meters<long long>>);
+  static_assert(std::is_same_v<std::common_type_t<kilometers<long long>, millimeters<float>>, millimeters<float>>);
 
-static_assert(std::is_same_v<std::common_type_t<meters<int>, kilometers<int>>, meters<int>>);
+  // quantity_cast
 
+  // static_assert(quantity_cast<int>(2_km).count() == 2000);  // should not compile
+  static_assert(quantity_cast<meters<int>>(2_km).count() == 2000);
+  static_assert(quantity_cast<kilometers<int>>(2000_m).count() == 2);
 
-// constructors
+  // time
 
-constexpr quantity<dimension_length, int> kilometer{1000};
-static_assert(kilometer.count() == 1000);
-static_assert(length<int>(kilometer).count() == kilometer.count());
-static_assert(meters<int>(kilometer).count() == kilometer.count());
-static_assert(millimeters<int>(kilometer).count() == kilometer.count() * 1000);
-static_assert(quantity_cast<kilometers<int>>(kilometer).count() == kilometer.count() / 1000);
+  //  static_assert(1_s == 1_m);  // should not compile
+  static_assert(1_h == 3600_s);
 
-//static_assert(meters<int>(1.0) == 1_m);   // should not compile
-static_assert(meters<int>(1) == 1_m);
-static_assert(meters<float>(1.0) == 1_m);
-static_assert(meters<float>(1) == 1_m);
+  // length
 
-//static_assert(kilometers<int>(1000_m) == 1000_m);   // should not compile
-static_assert(kilometers<float>(1000_m) == 1000_m);
-//static_assert(meters<int>(meters<float>(1)) == 1_m);  // should not compile
-static_assert(meters<int>(quantity_cast<meters<int>>(meters<float>(1))) == 1_m);
+  static_assert(1_km == 1000_m);
+  static_assert(1_km + 1_m == 1001_m);
+  static_assert(10_km / 5_km == 2);
+  static_assert(10_km / 2 == 5_km);
 
+  // velocity
 
-// zero(), min(), max()
+  static_assert(10_m / 5_s == 2_mps);
+  static_assert(10 / 5_s * 1_m == 2_mps);
+  static_assert(1_km / 1_s == 1000_mps);
+  static_assert(2_kmph * 2_h == 4_km);
+  static_assert(2_km / 2_kmph == 1_h);
 
-static_assert(length<int>::zero().count() == 0);
-static_assert(length<int>::min().count() == std::numeric_limits<int>::min());
-static_assert(length<int>::max().count() == std::numeric_limits<int>::max());
-
-
-// unary member operators
-
-static_assert((+1_m).count() == 1);
-static_assert((-1_m).count() == -1);
-
-
-// binary member operators
-
-template<typename Dimension, typename Rep, class Ratio = std::ratio<1>>
-constexpr auto post_inc(quantity<Dimension, Rep, Ratio> v)
-{
-  auto vv = v++;
-  return std::make_pair(v, vv);
-}
-
-template<typename Dimension, typename Rep, class Ratio = std::ratio<1>>
-constexpr auto pre_inc(quantity<Dimension, Rep, Ratio> v)
-{
-  auto vv = ++v;
-  return std::make_pair(v, vv);
-}
-
-template<typename Dimension, typename Rep, class Ratio = std::ratio<1>>
-constexpr auto post_dec(quantity<Dimension, Rep, Ratio> v)
-{
-  auto vv = v--;
-  return std::make_pair(v, vv);
-}
-
-template<typename Dimension, typename Rep, class Ratio = std::ratio<1>>
-constexpr auto pre_dec(quantity<Dimension, Rep, Ratio> v)
-{
-  auto vv = --v;
-  return std::make_pair(v, vv);
-}
-
-constexpr auto r1 = post_inc(1_m);
-static_assert(r1.first.count() == 2);
-static_assert(r1.second.count() == 1);
-
-constexpr auto r2 = pre_inc(1_m);
-static_assert(r2.first.count() == 2);
-static_assert(r2.second.count() == 2);
-
-constexpr auto r3 = post_dec(1_m);
-static_assert(r3.first.count() == 0);
-static_assert(r3.second.count() == 1);
-
-constexpr auto r4 = pre_dec(1_m);
-static_assert(r4.first.count() == 0);
-static_assert(r4.second.count() == 0);
-
-
-// compound assignment
-
-static_assert((1_km += 1_km).count() == 2);
-static_assert((2_km -= 1_km).count() == 1);
-static_assert((1_km *= 2).count() == 2);
-static_assert((2_km /= 2).count() == 1);
-static_assert((7_km %= 2).count() == 1);
-static_assert((7_km %= 2_km).count() == 1);
-
-
-// non-member arithmetic operators
-
-static_assert((1_km + 1_m).count() == 1001);
-static_assert((1_km - 1_m).count() == 999);
-static_assert((2_km * 2).count() == 4);
-static_assert((3 * 3_km).count() == 9);
-static_assert((2_kmph * 2_h).count() == 4);
-//static_assert(kilometers<float>(2_kmph * 15_min).count() == 500_m); // should not compile
-static_assert(kilometers<float>(2_kmph * 120.0_min).count() == 4);
-static_assert(kilometers<float>(2.0_kmph * 120_min).count() == 4);
-static_assert((4_km / 2).count() == 2);
-static_assert(4_km / 2_km == 2);
-static_assert((1_km / 1_s).count() == 1);
-//static_assert((1_km / 1_h).count() == 1);  // should not compile
-static_assert((1.0_km / 1_h).count() == 1);
-static_assert((7_km % 2).count() == 1);
-static_assert((7_km % 2_km).count() == 1);
-
-
-// comparators
-
-static_assert(2_km + 1_km == 3_km);
-static_assert(!(2_km + 2_km == 3_km));
-static_assert(2_km + 2_km != 3_km);
-static_assert(!(2_km + 2_km != 4_km));
-static_assert(2_km > 1_km);
-static_assert(!(1_km > 1_km));
-static_assert(1_km < 2_km);
-static_assert(!(2_km < 2_km));
-static_assert(2_km >= 1_km);
-static_assert(2_km >= 2_km);
-static_assert(!(2_km >= 3_km));
-static_assert(1_km <= 2_km);
-static_assert(2_km <= 2_km);
-static_assert(!(3_km <= 2_km));
-
-
-
-// quantity_cast
-
-static_assert(quantity_cast<kilometers<int>>(2000_m) == 2_km);
-// static_assert(quantity_cast<seconds<int>>(2_m) == 2_s);  // should not compile
+}  // namespace
diff --git a/test/test_tools.cpp b/test/test_tools.cpp
new file mode 100644
index 00000000..46e3f426
--- /dev/null
+++ b/test/test_tools.cpp
@@ -0,0 +1,47 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2018 Mateusz Pusz
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#include "units/si/velocity.h"
+#include <utility>
+
+namespace {
+
+  // static_sign
+
+  static_assert(mp::static_sign<2>::value == 1);
+  static_assert(mp::static_sign<-3>::value == -1);
+  static_assert(mp::static_sign<0>::value == 1);
+
+  // static_abs
+
+  static_assert(mp::static_abs<2>::value == 2);
+  static_assert(mp::static_abs<-3>::value == 3);
+  static_assert(mp::static_abs<0>::value == 0);
+
+  // common_ratio
+
+  static_assert(std::is_same_v<mp::common_ratio_t<std::ratio<1>, std::kilo>, std::ratio<1>>);
+  static_assert(std::is_same_v<mp::common_ratio_t<std::kilo, std::ratio<1>>, std::ratio<1>>);
+  static_assert(std::is_same_v<mp::common_ratio_t<std::ratio<1>, std::milli>, std::milli>);
+  static_assert(std::is_same_v<mp::common_ratio_t<std::milli, std::ratio<1>>, std::milli>);
+
+}  // namespace
\ No newline at end of file
diff --git a/test/test_type_list.cpp b/test/test_type_list.cpp
index f2fac0ed..cb14cbc4 100644
--- a/test/test_type_list.cpp
+++ b/test/test_type_list.cpp
@@ -21,57 +21,100 @@
 // SOFTWARE.
 
 #include "units/dimension.h"
-#include <gtest/gtest.h>
 #include <utility>
 
-using namespace mp;
-using namespace units;
+namespace {
 
-// type_list_push_front
+  using namespace mp;
+  using namespace units;
 
-static_assert(std::is_same_v<type_list_push_front_t<type_list<>, int>, type_list<int>>);
-static_assert(std::is_same_v<type_list_push_front_t<type_list<>, int, long, double>, type_list<int, long, double>>);
-static_assert(std::is_same_v<type_list_push_front_t<type_list<double>, int, long>, type_list<int, long, double>>);
+  // type_list_push_front
 
-// type_list_split_half
+  static_assert(std::is_same_v<type_list_push_front_t<type_list<>, int>, type_list<int>>);
+  static_assert(std::is_same_v<type_list_push_front_t<type_list<>, int, long, double>, type_list<int, long, double>>);
+  static_assert(std::is_same_v<type_list_push_front_t<type_list<double>, int, long>, type_list<int, long, double>>);
 
-static_assert(std::is_same_v<type_list_split_half<type_list<int>>::first_list, type_list<int>>);
-static_assert(std::is_same_v<type_list_split_half<type_list<int>>::second_list, type_list<>>);
+  // type_list_push_back
 
-static_assert(std::is_same_v<type_list_split_half<type_list<int, long>>::first_list, type_list<int>>);
-static_assert(std::is_same_v<type_list_split_half<type_list<int, long>>::second_list, type_list<long>>);
+  static_assert(std::is_same_v<type_list_push_back_t<type_list<>, int>, type_list<int>>);
+  static_assert(std::is_same_v<type_list_push_back_t<type_list<>, int, long, double>, type_list<int, long, double>>);
+  static_assert(std::is_same_v<type_list_push_back_t<type_list<double>, int, long>, type_list<double, int, long>>);
 
-static_assert(std::is_same_v<type_list_split_half<type_list<int, long, double>>::first_list, type_list<int, long>>);
-static_assert(std::is_same_v<type_list_split_half<type_list<int, long, double>>::second_list, type_list<double>>);
+  // type_list_split
 
-static_assert(
-    std::is_same_v<type_list_split_half<type_list<int, long, double, float>>::first_list, type_list<int, long>>);
-static_assert(
-    std::is_same_v<type_list_split_half<type_list<int, long, double, float>>::second_list, type_list<double, float>>);
+  static_assert(std::is_same_v<type_list_split<type_list<int>, 0>::first_list, type_list<>>);
+  static_assert(std::is_same_v<type_list_split<type_list<int>, 0>::second_list, type_list<int>>);
 
-// type_list_merge_sorted
+  static_assert(std::is_same_v<type_list_split<type_list<int>, 1>::first_list, type_list<int>>);
+  static_assert(std::is_same_v<type_list_split<type_list<int>, 1>::second_list, type_list<>>);
 
-static_assert(std::is_same_v<type_list_merge_sorted_t<type_list<dim_id<0>>, type_list<dim_id<1>>, dim_id_less>,
-                             type_list<dim_id<0>, dim_id<1>>>);
-static_assert(std::is_same_v<type_list_merge_sorted_t<type_list<dim_id<1>>, type_list<dim_id<0>>, dim_id_less>,
-                             type_list<dim_id<0>, dim_id<1>>>);
+  static_assert(std::is_same_v<type_list_split<type_list<int, long>, 0>::first_list, type_list<>>);
+  static_assert(std::is_same_v<type_list_split<type_list<int, long>, 0>::second_list, type_list<int, long>>);
 
-static_assert(std::is_same_v<type_list_merge_sorted_t<type_list<dim_id<27>, dim_id<38>>,
-                                                      type_list<dim_id<3>, dim_id<43>>, dim_id_less>,
-                             type_list<dim_id<3>, dim_id<27>, dim_id<38>, dim_id<43>>>);
-static_assert(
-    std::is_same_v<type_list_merge_sorted_t<type_list<dim_id<9>, dim_id<82>>, type_list<dim_id<10>>, dim_id_less>,
-                   type_list<dim_id<9>, dim_id<10>, dim_id<82>>>);
+  static_assert(std::is_same_v<type_list_split<type_list<int, long>, 1>::first_list, type_list<int>>);
+  static_assert(std::is_same_v<type_list_split<type_list<int, long>, 1>::second_list, type_list<long>>);
 
-// type_list_sort
+  static_assert(std::is_same_v<type_list_split<type_list<int, long>, 2>::first_list, type_list<int, long>>);
+  static_assert(std::is_same_v<type_list_split<type_list<int, long>, 2>::second_list, type_list<>>);
 
-static_assert(std::is_same_v<type_list_sort_t<type_list<dim_id<0>>, dim_id_less>, type_list<dim_id<0>>>);
-static_assert(
-    std::is_same_v<type_list_sort_t<type_list<dim_id<0>, dim_id<1>>, dim_id_less>, type_list<dim_id<0>, dim_id<1>>>);
-static_assert(
-    std::is_same_v<type_list_sort_t<type_list<dim_id<1>, dim_id<0>>, dim_id_less>, type_list<dim_id<0>, dim_id<1>>>);
-static_assert(
-    std::is_same_v<
-        type_list_sort_t<type_list<dim_id<38>, dim_id<27>, dim_id<43>, dim_id<3>, dim_id<9>, dim_id<82>, dim_id<10>>,
-                         dim_id_less>,
-        type_list<dim_id<3>, dim_id<9>, dim_id<10>, dim_id<27>, dim_id<38>, dim_id<43>, dim_id<82>>>);
+  static_assert(std::is_same_v<type_list_split<type_list<int, long, double>, 1>::first_list, type_list<int>>);
+  static_assert(std::is_same_v<type_list_split<type_list<int, long, double>, 1>::second_list, type_list<long, double>>);
+
+  static_assert(std::is_same_v<type_list_split<type_list<int, long, double>, 2>::first_list, type_list<int, long>>);
+  static_assert(std::is_same_v<type_list_split<type_list<int, long, double>, 2>::second_list, type_list<double>>);
+
+  // type_list_split_half
+
+  static_assert(std::is_same_v<type_list_split_half<type_list<int>>::first_list, type_list<int>>);
+  static_assert(std::is_same_v<type_list_split_half<type_list<int>>::second_list, type_list<>>);
+
+  static_assert(std::is_same_v<type_list_split_half<type_list<int, long>>::first_list, type_list<int>>);
+  static_assert(std::is_same_v<type_list_split_half<type_list<int, long>>::second_list, type_list<long>>);
+
+  static_assert(std::is_same_v<type_list_split_half<type_list<int, long, double>>::first_list, type_list<int, long>>);
+  static_assert(std::is_same_v<type_list_split_half<type_list<int, long, double>>::second_list, type_list<double>>);
+
+  static_assert(
+      std::is_same_v<type_list_split_half<type_list<int, long, double, float>>::first_list, type_list<int, long>>);
+  static_assert(
+      std::is_same_v<type_list_split_half<type_list<int, long, double, float>>::second_list, type_list<double, float>>);
+
+  // type_list_merge_sorted
+
+  static_assert(std::is_same_v<type_list_merge_sorted_t<type_list<dim_id<0>>, type_list<dim_id<1>>, dim_id_less>,
+                               type_list<dim_id<0>, dim_id<1>>>);
+  static_assert(std::is_same_v<type_list_merge_sorted_t<type_list<dim_id<1>>, type_list<dim_id<0>>, dim_id_less>,
+                               type_list<dim_id<0>, dim_id<1>>>);
+
+  static_assert(std::is_same_v<type_list_merge_sorted_t<type_list<dim_id<27>, dim_id<38>>,
+                                                        type_list<dim_id<3>, dim_id<43>>, dim_id_less>,
+                               type_list<dim_id<3>, dim_id<27>, dim_id<38>, dim_id<43>>>);
+  static_assert(
+      std::is_same_v<type_list_merge_sorted_t<type_list<dim_id<9>, dim_id<82>>, type_list<dim_id<10>>, dim_id_less>,
+                     type_list<dim_id<9>, dim_id<10>, dim_id<82>>>);
+
+  // type_list_sort
+
+  template<typename TypeList>
+  using dim_sort_t = type_list_sort_t<TypeList, dim_id_less>;
+
+  static_assert(std::is_same_v<dim_sort_t<type_list<dim_id<0>>>, type_list<dim_id<0>>>);
+  static_assert(std::is_same_v<dim_sort_t<type_list<dim_id<0>, dim_id<1>>>, type_list<dim_id<0>, dim_id<1>>>);
+  static_assert(std::is_same_v<dim_sort_t<type_list<dim_id<1>, dim_id<0>>>, type_list<dim_id<0>, dim_id<1>>>);
+  static_assert(std::is_same_v<
+                dim_sort_t<type_list<dim_id<38>, dim_id<27>, dim_id<43>, dim_id<3>, dim_id<9>, dim_id<82>, dim_id<10>>>,
+                type_list<dim_id<3>, dim_id<9>, dim_id<10>, dim_id<27>, dim_id<38>, dim_id<43>, dim_id<82>>>);
+
+  // exp_less
+
+  template<int Id, int Value>
+  using e = exp<dim_id<Id>, Value>;
+
+  template<typename TypeList>
+  using exp_sort_t = type_list_sort_t<TypeList, exp_less>;
+
+  static_assert(std::is_same_v<exp_sort_t<dimension<e<0, 1>>>, dimension<e<0, 1>>>);
+  static_assert(std::is_same_v<exp_sort_t<dimension<e<0, 1>, e<1, -1>>>, dimension<e<0, 1>, e<1, -1>>>);
+  static_assert(std::is_same_v<exp_sort_t<dimension<e<1, 1>, e<0, -1>>>, dimension<e<0, -1>, e<1, 1>>>);
+
+}  // namespace
diff --git a/test/test_units.cpp b/test/test_units.cpp
index 27a44284..6b7c67f7 100644
--- a/test/test_units.cpp
+++ b/test/test_units.cpp
@@ -20,51 +20,51 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+#include "units/si/frequency.h"
 #include "units/si/length.h"
 #include "units/si/time.h"
-#include "units/si/frequency.h"
 #include "units/si/velocity.h"
-#include <gtest/gtest.h>
 #include <utility>
 
-using namespace units;
-using namespace units::literals;
+namespace {
+  using namespace units;
+  using namespace units::literals;
 
+  // frequency
 
-// frequency
+  static_assert(2 / 1_s == 2_Hz);
+  static_assert(1000 / 1_s == 1_kHz);
+  static_assert(3.2_GHz == 3'200'000'000_Hz);
 
-static_assert(2 / 1_s == 2_Hz);
-static_assert(1000 / 1_s == 1_kHz);
-static_assert(3.2_GHz == 3'200'000'000_Hz);
+  // time
 
+  static_assert(1_h == 3600_s);
 
-// time
+  // length
 
-static_assert(1_h == 3600_s);
+  static_assert(1_km == 1000_m);
+  static_assert(1_km + 1_m == 1001_m);
+  static_assert(10_km / 5_km == 2);
+  static_assert(10_km / 2 == 5_km);
 
+  // velocity
 
-// length
+  static_assert(std::is_same_v<decltype(1_km / 1_s), velocity<long long int, std::ratio<1000, 1>>>);
 
-static_assert(1_km == 1000_m);
-static_assert(10_km / 5_km == 2);
-static_assert(10_km / 2 == 5_km);
+  static_assert(10_m / 5_s == 2_mps);
+  static_assert(10 / 5_s * 1_m == 2_mps);
+  static_assert(1_km / 1_s == 1000_mps);
+  // static_assert(1_km / 1_h == 1_kmph);  // should not compile
+  static_assert(1.0_km / 1_h == 1_kmph);
+  static_assert(1000.0_m / 3600.0_s == 1_kmph);
 
+  static_assert(2_kmph * 2_h == 4_km);
+  // static_assert(2_kmph * 15_min == 500_m); // should not compile
+  static_assert(2_kmph * 15.0_min == 500_m);
+  static_assert(2.0_kmph * 15_min == 500_m);
 
-// velocity
+  static_assert(2_km / 2_kmph == 1_h);
+  // static_assert(2000_m / 2_kmph == 1_h); // should not compile
+  static_assert(quantity_cast<kilometers<int>>(2000_m) / 2_kmph == 1_h);
 
-static_assert(std::is_same_v<decltype(1_km / 1_s), velocity<long long int, std::ratio<1000, 1>>>);
-
-static_assert(10_m / 5_s == 2_mps);
-static_assert(1_km / 1_s == 1000_mps);
-//static_assert(1_km / 1_h == 1_kmph);  // should not compile
-static_assert(1.0_km / 1_h == 1_kmph);
-static_assert(1000.0_m / 3600.0_s == 1_kmph);
-
-static_assert(2_kmph * 2_h == 4_km);
-//static_assert(2_kmph * 15_min == 500_m); // should not compile
-static_assert(2_kmph * 15.0_min == 500_m);
-static_assert(2.0_kmph * 15_min == 500_m);
-
-static_assert(2_km / 2_kmph == 1_h);
-// static_assert(2000_m / 2_kmph == 1_h); // should not compile
-static_assert(quantity_cast<kilometers<int>>(2000_m) / 2_kmph == 1_h);
+}  // namespace