diff --git a/boost_multiprecision.props b/boost_multiprecision.props
index cc3a72c69c..99870093f6 100644
--- a/boost_multiprecision.props
+++ b/boost_multiprecision.props
@@ -6,6 +6,7 @@
   <ItemDefinitionGroup>
     <ClCompile>
       <AdditionalIncludeDirectories>$(SolutionDir)..\Boost\multiprecision\include;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
+      <PreprocessorDefinitions>BOOST_MP_STANDALONE;%(PreprocessorDefinitions)</PreprocessorDefinitions>
     </ClCompile>
   </ItemDefinitionGroup>
   <ItemGroup />
diff --git a/functions/functions.vcxproj b/functions/functions.vcxproj
index 760525bf4f..766d73c873 100644
--- a/functions/functions.vcxproj
+++ b/functions/functions.vcxproj
@@ -4,11 +4,6 @@
     <ProjectGuid>{7cca653c-2e8f-4ffd-9e9f-bee590f3efab}</ProjectGuid>
     <RootNamespace>functions</RootNamespace>
   </PropertyGroup>
-  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Release|x64'">
-    <ClCompile>
-      <PreprocessorDefinitions>BOOST_MP_STANDALONE;%(PreprocessorDefinitions)</PreprocessorDefinitions>
-    </ClCompile>
-  </ItemDefinitionGroup>
   <ItemGroup>
     <ClInclude Include="accurate_table_generator.hpp" />
     <ClInclude Include="accurate_table_generator_body.hpp" />
diff --git a/numerics/polynomial_in_monomial_basis_body.hpp b/numerics/polynomial_in_monomial_basis_body.hpp
index 57624f058f..7403488ad8 100644
--- a/numerics/polynomial_in_monomial_basis_body.hpp
+++ b/numerics/polynomial_in_monomial_basis_body.hpp
@@ -12,7 +12,7 @@
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_join.h"
 #include "base/not_constructible.hpp"
-#include "boost/multiprecision/fwd.hpp"
+#include "boost/multiprecision/number.hpp"
 #include "geometry/cartesian_product.hpp"
 #include "geometry/serialization.hpp"
 #include "numerics/combinatorics.hpp"
diff --git a/quantities/concepts.hpp b/quantities/concepts.hpp
index 580487434b..6b7721fabb 100644
--- a/quantities/concepts.hpp
+++ b/quantities/concepts.hpp
@@ -15,6 +15,8 @@ using namespace base::_traits;
 using namespace quantities::_quantities;
 
 // TODO(egg): additive_group should subsume affine, but we use it there.
+// We use |convertible_to| here because we want this concept to work with
+// Boost multiprecision types which heavily use implicit conversions.
 template<typename G>
 concept additive_group = requires(G x, G y, int n) {
   G{};
@@ -34,10 +36,10 @@ concept additive_group = requires(G x, G y, int n) {
 template<typename A>
 concept affine = requires(A x, A y) {
   { x - y } -> additive_group;
-  { y + (x - y) } -> std::convertible_to<A>;
-  { y += (x - y) } -> std::convertible_to<A&>;
-  { y - (x - y) } -> std::convertible_to<A>;
-  { y -= (x - y) } -> std::convertible_to<A&>;
+  { y + (x - y) } -> std::same_as<A>;
+  { y += (x - y) } -> std::same_as<A&>;
+  { y - (x - y) } -> std::same_as<A>;
+  { y -= (x - y) } -> std::same_as<A&>;
 };
 
 // A graded ring restricted to its homogeneous elements; multiplication can
@@ -48,13 +50,13 @@ concept homogeneous_ring =
       // Really, multiplication should return a homogeneous_ring.
       { x * y } -> additive_group;
       { (x * y) * z } -> additive_group;
-      { (x * y) * z } -> std::convertible_to<decltype(x * (y * z))>;
+      { (x * y) * z } -> std::same_as<decltype(x * (y * z))>;
 };
 
 template<typename A>
 concept ring = homogeneous_ring<A> && requires(A x, A y) {
-  { x * y } -> std::convertible_to<A>;
-  { x *= y } -> std::convertible_to<A&>;
+  { x * y } -> std::same_as<A>;
+  { x *= y } -> std::same_as<A&>;
 };
 
 // TODO(egg): field should subsume homogeneous_field, but we use it in
@@ -63,9 +65,9 @@ concept ring = homogeneous_ring<A> && requires(A x, A y) {
 template<typename K>
 concept field = ring<K> && !std::integral<K> && requires(K x, K y, K z) {
   { 1 } -> std::convertible_to<K>;
-  { 1 / y } -> std::convertible_to<K>;
-  { x / y } -> std::convertible_to<K>;
-  { x /= y } -> std::convertible_to<K&>;
+  { 1 / y } -> std::same_as<K>;
+  { x / y } -> std::same_as<K>;
+  { x /= y } -> std::same_as<K&>;
 };
 
 // TODO(egg): vector_space should subsume homogeneous_vector_space, but we use
@@ -73,11 +75,11 @@ concept field = ring<K> && !std::integral<K> && requires(K x, K y, K z) {
 
 template<typename V, typename K>
 concept vector_space = field<K> && requires(K λ, V v) {
-      { λ * v } -> std::convertible_to<V>;
-      { v * λ } -> std::convertible_to<V>;
-      { v / λ } -> std::convertible_to<V>;
-      { v *= λ } -> std::convertible_to<V&>;
-      { v /= λ } -> std::convertible_to<V&>;
+      { λ * v } -> std::same_as<V>;
+      { v * λ } -> std::same_as<V>;
+      { v / λ } -> std::same_as<V>;
+      { v *= λ } -> std::same_as<V&>;
+      { v /= λ } -> std::same_as<V&>;
     };
 
 // A graded field restricted to its homogeneous elements; multiplication and
@@ -88,9 +90,9 @@ concept homogeneous_field = homogeneous_ring<K> && requires(K x, K y, K z) {
   { x / y } -> field;
   requires vector_space<K, decltype(x / y)>;
   { (1 / x) } -> homogeneous_ring;
-  { 1 / (x * y) } -> std::convertible_to<decltype((1 / x) * (1 / y))>;
-  { (x * y) / z } -> std::convertible_to<K>;
-  { (x / y) * z } -> std::convertible_to<K>;
+  { 1 / (x * y) } -> std::same_as<decltype((1 / x) * (1 / y))>;
+  { (x * y) / z } -> std::same_as<K>;
+  { (x / y) * z } -> std::same_as<K>;
 };
 
 template<typename V, typename K>
@@ -99,11 +101,11 @@ concept homogeneous_vector_space =
       // Really, these operations should return a homogeneous_vector_space.
       requires vector_space<V, decltype(λ / μ)>;
       { λ * v } -> additive_group;
-      { v * λ } -> std::convertible_to<decltype(λ * v)>;
+      { v * λ } -> std::same_as<decltype(λ * v)>;
       { v / λ } -> additive_group;
-      { λ * v / λ } -> std::convertible_to<V>;
+      { λ * v / λ } -> std::same_as<V>;
       { (λ * μ) * v } -> additive_group;
-      { λ * (μ * v) } -> std::convertible_to<decltype((λ * μ) * v)>;
+      { λ * (μ * v) } -> std::same_as<decltype((λ * μ) * v)>;
     };
 
 template<typename V>
@@ -118,7 +120,7 @@ template<typename V>
 concept real_affine_space = affine_space<V, double>;
 
 template<typename T>
-concept quantity = instance<T, Quantity> || std::convertible_to<T, double>;
+concept quantity = instance<T, Quantity> || std::same_as<T, double>;
 
 // std::integral || std::floating_point rather than
 // std::convertible_to<double, T> because
diff --git a/quantities/elementary_functions.hpp b/quantities/elementary_functions.hpp
index 4e2d6b817d..0a09449bb6 100644
--- a/quantities/elementary_functions.hpp
+++ b/quantities/elementary_functions.hpp
@@ -2,7 +2,7 @@
 
 #include <concepts>
 
-#include "boost/multiprecision/fwd.hpp"
+#include "boost/multiprecision/number.hpp"
 #include "quantities/named_quantities.hpp"
 #include "quantities/quantities.hpp"
 
diff --git a/quantities/elementary_functions_body.hpp b/quantities/elementary_functions_body.hpp
index f1cd9ab58e..0da6aa541c 100644
--- a/quantities/elementary_functions_body.hpp
+++ b/quantities/elementary_functions_body.hpp
@@ -7,7 +7,6 @@
 #include <cmath>
 
 #include "boost/multiprecision/cpp_int.hpp"
-#include "boost/multiprecision/fwd.hpp"
 #include "numerics/cbrt.hpp"
 #include "numerics/fma.hpp"
 #include "numerics/next.hpp"
diff --git a/quantities/generators_body.hpp b/quantities/generators_body.hpp
index edc77f07bd..1b214ee96a 100644
--- a/quantities/generators_body.hpp
+++ b/quantities/generators_body.hpp
@@ -5,7 +5,7 @@
 #include <tuple>
 
 #include "base/not_constructible.hpp"
-#include "boost/multiprecision/fwd.hpp"
+#include "boost/multiprecision/number.hpp"
 #include "quantities/dimensions.hpp"
 
 namespace principia {
diff --git a/quantities/quantities.hpp b/quantities/quantities.hpp
index f9380a86de..a1e46eecb4 100644
--- a/quantities/quantities.hpp
+++ b/quantities/quantities.hpp
@@ -12,7 +12,7 @@
 #include "base/not_constructible.hpp"
 #include "base/not_null.hpp"
 #include "base/tags.hpp"
-#include "boost/multiprecision/fwd.hpp"
+#include "boost/multiprecision/number.hpp"
 #include "quantities/dimensions.hpp"
 #include "quantities/generators.hpp"
 #include "serialization/quantities.pb.h"