diff --git a/src/core/include/mp-units/bits/fmt.h b/src/core/include/mp-units/bits/fmt.h
index 186f0ae2..eee5622a 100644
--- a/src/core/include/mp-units/bits/fmt.h
+++ b/src/core/include/mp-units/bits/fmt.h
@@ -93,7 +93,7 @@ struct fmt_arg_ref {
 template<typename Char>
 struct fill_t {
 private:
-  static constexpr size_t max_size = 4 / sizeof(Char);
+  static constexpr std::size_t max_size = 4 / sizeof(Char);
   // At most one codepoint (so one char32_t or four utf-8 char8_t)
   std::array<Char, max_size> data_ = {Char{' '}};
   unsigned char size_ = 1;
@@ -103,16 +103,16 @@ public:
   {
     auto size = str.size();
     if (size > max_size) MP_UNITS_THROW(MP_UNITS_STD_FMT::format_error("invalid fill"));
-    for (size_t i = 0; i < size && i < max_size; ++i) data_[i] = str[i];
+    for (std::size_t i = 0; i < size && i < max_size; ++i) data_[i] = str[i];
     size_ = static_cast<unsigned char>(size);
     return *this;
   }
 
-  [[nodiscard]] constexpr size_t size() const { return size_; }
+  [[nodiscard]] constexpr std::size_t size() const { return size_; }
   [[nodiscard]] constexpr const Char* data() const { return data_.data(); }
 
-  [[nodiscard]] constexpr Char& operator[](size_t index) { return data_[index]; }
-  [[nodiscard]] constexpr const Char& operator[](size_t index) const { return data_[index]; }
+  [[nodiscard]] constexpr Char& operator[](std::size_t index) { return data_[index]; }
+  [[nodiscard]] constexpr const Char& operator[](std::size_t index) const { return data_[index]; }
 };
 
 MP_UNITS_EXPORT_END
diff --git a/src/core/include/mp-units/ext/fixed_string.h b/src/core/include/mp-units/ext/fixed_string.h
index ca4d8284..2bcc45e0 100644
--- a/src/core/include/mp-units/ext/fixed_string.h
+++ b/src/core/include/mp-units/ext/fixed_string.h
@@ -214,33 +214,33 @@ public:
     MP_UNITS_EXPECTS(lhs[N1 - 1] == CharT{});
     CharT txt[N1 + N];
     CharT* it = txt;
-    for (size_t i = 0; i != N1 - 1; ++i) *it++ = lhs[i];
+    for (std::size_t i = 0; i != N1 - 1; ++i) *it++ = lhs[i];
     for (CharT c : rhs) *it++ = c;
     *it++ = CharT();
     return txt;
   }
 
   // non-member comparison functions
-  template<size_t N2>
+  template<std::size_t N2>
   [[nodiscard]] friend constexpr bool operator==(const basic_fixed_string& lhs,
                                                  const basic_fixed_string<CharT, N2, Traits>& rhs)
   {
     return lhs.view() == rhs.view();
   }
-  template<size_t N2>
+  template<std::size_t N2>
   [[nodiscard]] friend consteval bool operator==(const basic_fixed_string& lhs, const CharT (&rhs)[N2])
   {
     MP_UNITS_EXPECTS(rhs[N2 - 1] == CharT{});
     return lhs.view() == std::basic_string_view<CharT, Traits>(std::cbegin(rhs), std::cend(rhs) - 1);
   }
 
-  template<size_t N2>
+  template<std::size_t N2>
   [[nodiscard]] friend constexpr auto operator<=>(const basic_fixed_string& lhs,
                                                   const basic_fixed_string<CharT, N2, Traits>& rhs)
   {
     return lhs.view() <=> rhs.view();
   }
-  template<size_t N2>
+  template<std::size_t N2>
   [[nodiscard]] friend consteval auto operator<=>(const basic_fixed_string& lhs, const CharT (&rhs)[N2])
   {
     MP_UNITS_EXPECTS(rhs[N2 - 1] == CharT{});
@@ -268,7 +268,7 @@ template<one_of<char, char8_t, char16_t, char32_t, wchar_t> CharT, std::size_t N
 basic_fixed_string(std::from_range_t, std::array<CharT, N>) -> basic_fixed_string<CharT, N>;
 
 // specialized algorithms
-template<class CharT, size_t N, class Traits>
+template<class CharT, std::size_t N, class Traits>
 constexpr void swap(basic_fixed_string<CharT, N, Traits>& x, basic_fixed_string<CharT, N, Traits>& y) noexcept
 {
   x.swap(y);
diff --git a/src/core/include/mp-units/random.h b/src/core/include/mp-units/random.h
index 5736451d..145f7161 100644
--- a/src/core/include/mp-units/random.h
+++ b/src/core/include/mp-units/random.h
@@ -38,7 +38,7 @@ template<Quantity Q, typename InputIt>
 std::vector<typename Q::rep> i_qty_to_rep(InputIt first, InputIt last)
 {
   std::vector<typename Q::rep> intervals_rep;
-  intervals_rep.reserve(static_cast<size_t>(std::distance(first, last)));
+  intervals_rep.reserve(static_cast<std::size_t>(std::distance(first, last)));
   for (InputIt itr = first; itr != last; ++itr) {
     intervals_rep.push_back(itr->numerical_value_ref_in(Q::unit));
   }
@@ -67,7 +67,7 @@ std::vector<typename Q::rep> fw_bl_pwc(std::initializer_list<Q>& bl, UnaryOperat
   }
   std::vector<rep> weights;
   weights.reserve(bl.size());
-  for (size_t i = 0; i < bl.size() - 1; ++i) {
+  for (std::size_t i = 0; i < bl.size() - 1; ++i) {
     weights.push_back(w_bl[i] + w_bl[i + 1]);
   }
   weights.push_back(0);