diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -41,6 +41,7 @@
   __ranges/concepts.h
   __ranges/end.h
   __ranges/enable_borrowed_range.h
+  __ranges/size.h
   __split_buffer
   __sso_allocator
   __std_stream
diff --git a/libcxx/include/__ranges/size.h b/libcxx/include/__ranges/size.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__ranges/size.h
@@ -0,0 +1,125 @@
+// -*- C++ -*-
+//===------------------------ __ranges/end.h ------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+#ifndef _LIBCPP_RANGES_SIZE_H
+#define _LIBCPP_RANGES_SIZE_H
+
+#include <__config>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/concepts.h>
+#include <__ranges/begin.h>
+#include <__ranges/end.h>
+#include <type_traits>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if !defined(_LIBCPP_HAS_NO_RANGES)
+
+template<class, class>
+inline constexpr bool disable_sized_sentinel_for = false;
+
+template<class _Sp, class _Ip>
+concept sized_sentinel_for =
+  sentinel_for<_Sp, _Ip> &&
+  !disable_sized_sentinel_for<remove_cv_t<_Sp>, remove_cv_t<_Ip>> &&
+  requires(const _Ip& __i, const _Sp& __s) {
+    { __s - __i } -> same_as<iter_difference_t<_Ip>>;
+    { __i - __s } -> same_as<iter_difference_t<_Ip>>;
+  };
+
+template<class>
+inline constexpr bool disable_sized_range = false;
+
+// clang-format off
+namespace ranges {
+// [range.prim.size]
+namespace __size {
+
+void size(auto&) = delete;
+void size(const auto&) = delete;
+
+template <class _Tp>
+concept __size_enabled = !disable_sized_range<remove_cvref_t<_Tp>>;
+
+template <class _Tp>
+concept __member_size = __size_enabled<_Tp> && requires(_Tp&& __t) {
+  { _VSTD::__decay_copy(_VSTD::forward<_Tp>(__t).size()) } -> integral;
+};
+
+template <class _Tp>
+concept __unqualified_size =
+  __size_enabled<_Tp> &&
+  !__member_size<_Tp> &&
+  __class_or_enum<remove_cvref_t<_Tp>> &&
+  requires(_Tp&& __t) {
+    { _VSTD::__decay_copy(size(_VSTD::forward<_Tp>(__t))) } -> integral;
+  };
+
+template <class _Tp>
+concept __difference =
+  !__member_size<_Tp> &&
+  !__unqualified_size<_Tp> &&
+  __class_or_enum<remove_cvref_t<_Tp>> &&
+  requires(_Tp&& __t) {
+    { ranges::begin(__t) } -> forward_iterator;
+    { ranges::end(__t) } -> sized_sentinel_for<decltype(ranges::begin(declval<_Tp>()))>;
+  };
+
+struct __fn {
+  template <class _Tp, size_t _Sz>
+  [[nodiscard]] constexpr size_t operator()(_Tp (&&)[_Sz]) const noexcept {
+    return _Sz;
+  }
+
+  template <class _Tp, size_t _Sz>
+  [[nodiscard]] constexpr size_t operator()(_Tp (&)[_Sz]) const noexcept {
+    return _Sz;
+  }
+
+  template <__member_size _Tp>
+  [[nodiscard]] constexpr integral auto operator()(_Tp&& __t) const
+      noexcept(noexcept(_VSTD::forward<_Tp>(__t).size())) {
+    return _VSTD::forward<_Tp>(__t).size();
+  }
+
+  template <__unqualified_size _Tp>
+  [[nodiscard]] constexpr integral auto operator()(_Tp&& __t) const
+      noexcept(noexcept(size(_VSTD::forward<_Tp>(__t)))) {
+    return size(_VSTD::forward<_Tp>(__t));
+  }
+
+  template<__difference _Tp>
+  [[nodiscard]] constexpr integral auto operator()(_Tp&& __t) const
+      noexcept(noexcept(ranges::end(__t) - ranges::begin(__t))) {
+    using _Unsigned = make_unsigned_t<range_difference_t<remove_cvref_t<_Tp>>>;
+    return static_cast<_Unsigned>(ranges::end(__t) - ranges::begin(__t));
+  }
+};
+}
+
+inline namespace __cpo {
+inline constexpr const auto size = __size::__fn{};
+} // namespace __cpo
+} // namespace ranges
+
+// clang-format off
+
+#endif // !defined(_LIBCPP_HAS_NO_RANGES)
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP_RANGES_SIZE_H
diff --git a/libcxx/include/ranges b/libcxx/include/ranges
--- a/libcxx/include/ranges
+++ b/libcxx/include/ranges
@@ -61,6 +61,7 @@
 #include <__ranges/concepts.h>
 #include <__ranges/end.h>
 #include <__ranges/enable_borrowed_range.h>
+#include <__ranges/size.h>
 #include <compare>          // Required by the standard.
 #include <initializer_list> // Required by the standard.
 #include <iterator>         // Required by the standard.
diff --git a/libcxx/test/std/ranges/range.access/range.prim/size.pass.cpp b/libcxx/test/std/ranges/range.access/range.prim/size.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/ranges/range.access/range.prim/size.pass.cpp
@@ -0,0 +1,282 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+
+// std::ranges::size
+
+#include <ranges>
+
+#include <cassert>
+#include "test_macros.h"
+#include "test_iterators.h"
+
+using RangeSizeT = decltype(std::ranges::size);
+
+static_assert(!std::is_invocable_v<RangeSizeT, int[]>);
+static_assert( std::is_invocable_v<RangeSizeT, int[1]>);
+static_assert( std::is_invocable_v<RangeSizeT, int (&&)[1]>);
+static_assert( std::is_invocable_v<RangeSizeT, int (&)[1]>);
+
+struct SizeMember {
+  constexpr size_t size() { return 42; }
+};
+
+static_assert(!std::is_invocable_v<RangeSizeT, const SizeMember>);
+
+struct SizeFunction {
+  friend constexpr size_t size(SizeFunction) { return 42; }
+};
+
+// Make sure the size member is preferred.
+struct SizeMemberAndFunction {
+  constexpr size_t size() { return 42; }
+  friend constexpr size_t size(SizeMemberAndFunction) { return 0; }
+};
+
+bool constexpr testArrayType() {
+  int a[4];
+  int b[1];
+  SizeMember c[4];
+  SizeFunction d[4];
+
+  assert(std::ranges::size(a) == 4);
+  assert(std::ranges::size(b) == 1);
+  assert(std::ranges::size(c) == 4);
+  assert(std::ranges::size(d) == 4);
+
+  return true;
+}
+
+struct SizeMemberConst {
+  constexpr size_t size() const { return 42; }
+};
+
+struct SizeMemberSigned {
+  constexpr long size() { return 42; }
+};
+
+bool constexpr testHasSizeMember() {
+  assert(std::ranges::size(SizeMember()) == 42);
+
+  const SizeMemberConst sizeMemberConst;
+  assert(std::ranges::size(sizeMemberConst) == 42);
+
+  assert(std::ranges::size(SizeMemberAndFunction()) == 42);
+
+  assert(std::ranges::size(SizeMemberSigned()) == 42);
+  ASSERT_SAME_TYPE(decltype(std::ranges::size(SizeMemberSigned())), long);
+
+  return true;
+}
+
+struct MoveOnlySizeFunction {
+  MoveOnlySizeFunction() = default;
+  MoveOnlySizeFunction(MoveOnlySizeFunction &&) = default;
+  MoveOnlySizeFunction(MoveOnlySizeFunction const&) = delete;
+
+  friend constexpr size_t size(MoveOnlySizeFunction) { return 42; }
+};
+
+enum EnumSizeFunction {
+  a, b
+};
+
+constexpr size_t size(EnumSizeFunction) { return 42; }
+
+struct SizeFunctionConst {
+  friend constexpr size_t size(const SizeFunctionConst) { return 42; }
+};
+
+struct SizeFunctionRef {
+  friend constexpr size_t size(SizeFunctionRef&) { return 42; }
+};
+
+struct SizeFunctionConstRef {
+  friend constexpr size_t size(SizeFunctionConstRef const&) { return 42; }
+};
+
+struct SizeFunctionSigned {
+  friend constexpr long size(SizeFunctionSigned) { return 42; }
+};
+
+bool constexpr testHasSizeFunction() {
+  assert(std::ranges::size(SizeFunction()) == 42);
+  assert(std::ranges::size(MoveOnlySizeFunction()) == 42);
+  assert(std::ranges::size(EnumSizeFunction()) == 42);
+  assert(std::ranges::size(SizeFunctionConst()) == 42);
+
+  SizeFunctionRef a;
+  assert(std::ranges::size(a) == 42);
+
+  const SizeFunctionConstRef b;
+  assert(std::ranges::size(b) == 42);
+
+  assert(std::ranges::size(SizeFunctionSigned()) == 42);
+  ASSERT_SAME_TYPE(decltype(std::ranges::size(SizeFunctionSigned())), long);
+
+  return true;
+}
+
+struct Empty { };
+static_assert(!std::is_invocable_v<RangeSizeT, Empty>);
+
+struct InvalidReturnTypeMember {
+  Empty size();
+};
+
+struct InvalidReturnTypeFunction {
+  friend Empty size(InvalidReturnTypeFunction);
+};
+
+struct Convertible {
+  operator size_t();
+};
+
+struct ConvertibleReturnTypeMember {
+  Convertible size();
+};
+
+struct ConvertibleReturnTypeFunction {
+  friend Convertible size(ConvertibleReturnTypeFunction);
+};
+
+static_assert(!std::is_invocable_v<RangeSizeT, InvalidReturnTypeMember>);
+static_assert(!std::is_invocable_v<RangeSizeT, InvalidReturnTypeFunction>);
+static_assert( std::is_invocable_v<RangeSizeT, InvalidReturnTypeMember (&)[4]>);
+static_assert( std::is_invocable_v<RangeSizeT, InvalidReturnTypeFunction (&)[4]>);
+static_assert(!std::is_invocable_v<RangeSizeT, ConvertibleReturnTypeMember>);
+static_assert(!std::is_invocable_v<RangeSizeT, ConvertibleReturnTypeFunction>);
+
+struct SizeMemberDisabled {
+  size_t size() { return 42; }
+};
+
+template<>
+inline constexpr bool std::disable_sized_range<SizeMemberDisabled> = true;
+
+struct ConstSizeMemberDisabled {
+  size_t size() const { return 42; }
+};
+
+// Intentionally disabling "const ConstSizeMemberDisabled". This doesn't disable anything
+// because T is always uncvrefed before being checked.
+template<>
+inline constexpr bool std::disable_sized_range<ConstSizeMemberDisabled> = true;
+
+struct SizeFunctionDisabled {
+  friend size_t size(SizeFunctionDisabled) { return 42; }
+};
+
+template<>
+inline constexpr bool std::disable_sized_range<SizeFunctionDisabled> = true;
+
+struct ConstSizeFunctionDisabled {
+  friend size_t size(const ConstSizeFunctionDisabled) { return 42; }
+};
+
+template<>
+inline constexpr bool std::disable_sized_range<ConstSizeFunctionDisabled> = true;
+
+static_assert(!std::is_invocable_v<RangeSizeT, SizeMemberDisabled>);
+static_assert(!std::is_invocable_v<RangeSizeT, const ConstSizeMemberDisabled>);
+static_assert(!std::is_invocable_v<RangeSizeT, SizeFunctionDisabled>);
+static_assert(!std::is_invocable_v<RangeSizeT, const ConstSizeFunctionDisabled>);
+
+// No begin end.
+struct HasMinusOperator {
+  friend constexpr size_t operator-(HasMinusOperator, HasMinusOperator) { return 2; }
+};
+static_assert(!std::is_invocable_v<RangeSizeT, HasMinusOperator>);
+
+struct sentinel {
+  bool operator==(std::input_or_output_iterator auto) const { return true; }
+};
+
+struct HasMinusBeginEnd {
+  friend constexpr forward_iterator<int*> begin(HasMinusBeginEnd) { return {}; }
+  friend constexpr sentinel end(HasMinusBeginEnd) { return {}; }
+};
+
+constexpr long operator-(const sentinel, const forward_iterator<int*>) { return 2; }
+constexpr long operator-(const forward_iterator<int*>, const sentinel) { return 2; }
+
+struct other_forward_iterator : forward_iterator<int*> { };
+
+struct InvalidMinusBeginEnd {
+  friend constexpr other_forward_iterator begin(InvalidMinusBeginEnd) { return {}; }
+  friend constexpr sentinel end(InvalidMinusBeginEnd) { return {}; }
+};
+
+// Int is integer-like, but it is not other_forward_iterator's difference_type.
+constexpr int operator-(const sentinel, const other_forward_iterator) { return 2; }
+constexpr int operator-(const other_forward_iterator, const sentinel) { return 2; }
+
+static_assert(!std::same_as<other_forward_iterator::difference_type, int>);
+static_assert(!std::is_invocable_v<RangeSizeT, InvalidMinusBeginEnd>);
+
+struct BidirectionalRange {
+  constexpr random_access_iterator<int*> begin() { return {}; }
+  constexpr sentinel end() { return {}; }
+};
+
+constexpr long operator-(const sentinel, const random_access_iterator<int*>) { return 2; }
+constexpr long operator-(const random_access_iterator<int*>, const sentinel) { return 2; }
+
+struct IntPtrBeginAndEnd {
+  int buff[8];
+  constexpr int* begin() { return buff; }
+  constexpr int* end() { return buff + 8; }
+};
+
+struct DisabledSizeRangeWithBeginEnd {
+  int buff[8];
+  constexpr int* begin() { return buff; }
+  constexpr int* end() { return buff + 8; }
+  constexpr size_t size() { return 1; }
+};
+
+template<>
+inline constexpr bool std::disable_sized_range<DisabledSizeRangeWithBeginEnd> = true;
+
+constexpr bool testRanges() {
+  HasMinusBeginEnd a;
+  assert(std::ranges::size(a) == 2);
+  // Ensure that this is converted to an *unsigned* type.
+  ASSERT_SAME_TYPE(decltype(std::ranges::size(a)), unsigned long);
+
+  IntPtrBeginAndEnd b;
+  assert(std::ranges::size(b) == 8);
+
+  DisabledSizeRangeWithBeginEnd c;
+  assert(std::ranges::size(c) == 8);
+
+  BidirectionalRange d;
+  assert(std::ranges::size(d) == 2);
+  ASSERT_SAME_TYPE(decltype(std::ranges::size(d)), unsigned long);
+
+  return true;
+}
+
+int main(int, char**) {
+  testArrayType();
+  static_assert(testArrayType());
+
+  testHasSizeMember();
+  static_assert(testHasSizeMember());
+
+  testHasSizeFunction();
+  static_assert(testHasSizeFunction());
+
+  testRanges();
+  static_assert(testRanges());
+
+  return 0;
+}