diff --git a/au/code/au/CMakeLists.txt b/au/code/au/CMakeLists.txt
index a182fd30..ca41ca31 100644
--- a/au/code/au/CMakeLists.txt
+++ b/au/code/au/CMakeLists.txt
@@ -43,6 +43,13 @@ header_only_library(
     unit_symbol.hh
     wrapper_operations.hh
     zero.hh
+    constants/avogadro_constant.hh
+    constants/boltzmann_constant.hh
+    constants/cesium_hyperfine_transition_frequency.hh
+    constants/elementary_charge.hh
+    constants/luminous_efficacy_540_terahertz.hh
+    constants/planck_constant.hh
+    constants/reduced_planck_constant.hh
     constants/speed_of_light.hh
     stdx/experimental/is_detected.hh
     stdx/functional.hh
@@ -224,6 +231,13 @@ gtest_based_test(
 gtest_based_test(
   NAME constants_test
   SRCS
+    constants/test/avogadro_constant_test.cc
+    constants/test/boltzmann_constant_test.cc
+    constants/test/cesium_hyperfine_transition_frequency_test.cc
+    constants/test/elementary_charge_test.cc
+    constants/test/luminous_efficacy_540_terahertz_test.cc
+    constants/test/planck_constant_test.cc
+    constants/test/reduced_planck_constant_test.cc
     constants/test/speed_of_light_test.cc
   DEPS
     au
diff --git a/au/code/au/constants/avogadro_constant.hh b/au/code/au/constants/avogadro_constant.hh
new file mode 100644
index 00000000..c593fe19
--- /dev/null
+++ b/au/code/au/constants/avogadro_constant.hh
@@ -0,0 +1,37 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constant.hh"
+#include "au/units/moles.hh"
+
+namespace au {
+
+namespace detail {
+// DO NOT follow this pattern to define your own units.  This is for library-defined units.
+// Instead, follow instructions at (https://aurora-opensource.github.io/au/main/howto/new-units/).
+template <typename T>
+struct AvogadroConstantLabel {
+    static constexpr const char label[] = "N_A";
+};
+template <typename T>
+constexpr const char AvogadroConstantLabel<T>::label[];
+struct AvogadroConstantUnit : decltype(inverse(Moles{}) * mag<602'214'076>() * pow<15>(mag<10>())),
+                              AvogadroConstantLabel<void> {
+    using AvogadroConstantLabel<void>::label;
+};
+}  // namespace detail
+
+constexpr auto AVOGADRO_CONSTANT = make_constant(detail::AvogadroConstantUnit{});
+
+}  // namespace au
diff --git a/au/code/au/constants/boltzmann_constant.hh b/au/code/au/constants/boltzmann_constant.hh
new file mode 100644
index 00000000..8909a354
--- /dev/null
+++ b/au/code/au/constants/boltzmann_constant.hh
@@ -0,0 +1,39 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constant.hh"
+#include "au/units/joules.hh"
+#include "au/units/kelvins.hh"
+
+namespace au {
+
+namespace detail {
+// DO NOT follow this pattern to define your own units.  This is for library-defined units.
+// Instead, follow instructions at (https://aurora-opensource.github.io/au/main/howto/new-units/).
+template <typename T>
+struct BoltzmannConstantLabel {
+    static constexpr const char label[] = "k_B";
+};
+template <typename T>
+constexpr const char BoltzmannConstantLabel<T>::label[];
+struct BoltzmannConstantUnit
+    : decltype((Joules{} / Kelvins{}) * mag<1'380'649>() * pow<-29>(mag<10>())),
+      BoltzmannConstantLabel<void> {
+    using BoltzmannConstantLabel<void>::label;
+};
+}  // namespace detail
+
+constexpr auto BOLTZMANN_CONSTANT = make_constant(detail::BoltzmannConstantUnit{});
+
+}  // namespace au
diff --git a/au/code/au/constants/cesium_hyperfine_transition_frequency.hh b/au/code/au/constants/cesium_hyperfine_transition_frequency.hh
new file mode 100644
index 00000000..a09511e4
--- /dev/null
+++ b/au/code/au/constants/cesium_hyperfine_transition_frequency.hh
@@ -0,0 +1,38 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constant.hh"
+#include "au/units/hertz.hh"
+
+namespace au {
+
+namespace detail {
+// DO NOT follow this pattern to define your own units.  This is for library-defined units.
+// Instead, follow instructions at (https://aurora-opensource.github.io/au/main/howto/new-units/).
+template <typename T>
+struct CesiumHyperfineTransitionFrequencyLabel {
+    static constexpr const char label[] = "Delta_nu_Cs";
+};
+template <typename T>
+constexpr const char CesiumHyperfineTransitionFrequencyLabel<T>::label[];
+struct CesiumHyperfineTransitionFrequencyUnit : decltype(Hertz{} * mag<9'192'631'770>()),
+                                                CesiumHyperfineTransitionFrequencyLabel<void> {
+    using CesiumHyperfineTransitionFrequencyLabel<void>::label;
+};
+}  // namespace detail
+
+constexpr auto CESIUM_HYPERFINE_TRANSITION_FREQUENCY =
+    make_constant(detail::CesiumHyperfineTransitionFrequencyUnit{});
+
+}  // namespace au
diff --git a/au/code/au/constants/elementary_charge.hh b/au/code/au/constants/elementary_charge.hh
new file mode 100644
index 00000000..1b75a075
--- /dev/null
+++ b/au/code/au/constants/elementary_charge.hh
@@ -0,0 +1,37 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constant.hh"
+#include "au/units/coulombs.hh"
+
+namespace au {
+
+namespace detail {
+// DO NOT follow this pattern to define your own units.  This is for library-defined units.
+// Instead, follow instructions at (https://aurora-opensource.github.io/au/main/howto/new-units/).
+template <typename T>
+struct ElementaryChargeLabel {
+    static constexpr const char label[] = "e";
+};
+template <typename T>
+constexpr const char ElementaryChargeLabel<T>::label[];
+struct ElementaryChargeUnit : decltype(Coulombs{} * mag<1'602'176'634>() * pow<-28>(mag<10>())),
+                              ElementaryChargeLabel<void> {
+    using ElementaryChargeLabel<void>::label;
+};
+}  // namespace detail
+
+constexpr auto ELEMENTARY_CHARGE = make_constant(detail::ElementaryChargeUnit{});
+
+}  // namespace au
diff --git a/au/code/au/constants/luminous_efficacy_540_terahertz.hh b/au/code/au/constants/luminous_efficacy_540_terahertz.hh
new file mode 100644
index 00000000..adcc4826
--- /dev/null
+++ b/au/code/au/constants/luminous_efficacy_540_terahertz.hh
@@ -0,0 +1,39 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constant.hh"
+#include "au/units/lumens.hh"
+#include "au/units/watts.hh"
+
+namespace au {
+
+namespace detail {
+// DO NOT follow this pattern to define your own units.  This is for library-defined units.
+// Instead, follow instructions at (https://aurora-opensource.github.io/au/main/howto/new-units/).
+template <typename T>
+struct LuminousEfficacy540TerahertzLabel {
+    static constexpr const char label[] = "K_cd";
+};
+template <typename T>
+constexpr const char LuminousEfficacy540TerahertzLabel<T>::label[];
+struct LuminousEfficacy540TerahertzUnit : decltype((Lumens{} / Watts{}) * mag<683>()),
+                                          LuminousEfficacy540TerahertzLabel<void> {
+    using LuminousEfficacy540TerahertzLabel<void>::label;
+};
+}  // namespace detail
+
+constexpr auto LUMINOUS_EFFICACY_540_TERAHERTZ =
+    make_constant(detail::LuminousEfficacy540TerahertzUnit{});
+
+}  // namespace au
diff --git a/au/code/au/constants/planck_constant.hh b/au/code/au/constants/planck_constant.hh
new file mode 100644
index 00000000..8bf31d47
--- /dev/null
+++ b/au/code/au/constants/planck_constant.hh
@@ -0,0 +1,39 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constant.hh"
+#include "au/units/joules.hh"
+#include "au/units/seconds.hh"
+
+namespace au {
+
+namespace detail {
+// DO NOT follow this pattern to define your own units.  This is for library-defined units.
+// Instead, follow instructions at (https://aurora-opensource.github.io/au/main/howto/new-units/).
+template <typename T>
+struct PlanckConstantLabel {
+    static constexpr const char label[] = "h";
+};
+template <typename T>
+constexpr const char PlanckConstantLabel<T>::label[];
+struct PlanckConstantUnit
+    : decltype(Joules{} * Seconds{} * mag<662'607'015>() * pow<-42>(mag<10>())),
+      PlanckConstantLabel<void> {
+    using PlanckConstantLabel<void>::label;
+};
+}  // namespace detail
+
+constexpr auto PLANCK_CONSTANT = make_constant(detail::PlanckConstantUnit{});
+
+}  // namespace au
diff --git a/au/code/au/constants/reduced_planck_constant.hh b/au/code/au/constants/reduced_planck_constant.hh
new file mode 100644
index 00000000..5ada7977
--- /dev/null
+++ b/au/code/au/constants/reduced_planck_constant.hh
@@ -0,0 +1,39 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constant.hh"
+#include "au/units/joules.hh"
+#include "au/units/seconds.hh"
+
+namespace au {
+
+namespace detail {
+// DO NOT follow this pattern to define your own units.  This is for library-defined units.
+// Instead, follow instructions at (https://aurora-opensource.github.io/au/main/howto/new-units/).
+template <typename T>
+struct ReducedPlanckConstantLabel {
+    static constexpr const char label[] = "h_bar";
+};
+template <typename T>
+constexpr const char ReducedPlanckConstantLabel<T>::label[];
+struct ReducedPlanckConstantUnit : decltype(Joules{} * Seconds{} * mag<662'607'015>() *
+                                            pow<-42>(mag<10>()) / mag<2>() / Magnitude<Pi>{}),
+                                   ReducedPlanckConstantLabel<void> {
+    using ReducedPlanckConstantLabel<void>::label;
+};
+}  // namespace detail
+
+constexpr auto REDUCED_PLANCK_CONSTANT = make_constant(detail::ReducedPlanckConstantUnit{});
+
+}  // namespace au
diff --git a/au/code/au/constants/test/avogadro_constant_test.cc b/au/code/au/constants/test/avogadro_constant_test.cc
new file mode 100644
index 00000000..4f299b80
--- /dev/null
+++ b/au/code/au/constants/test/avogadro_constant_test.cc
@@ -0,0 +1,45 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constants/avogadro_constant.hh"
+
+#include "au/testing.hh"
+#include "au/units/moles.hh"
+#include "gtest/gtest.h"
+
+namespace au {
+namespace {
+
+using ::testing::StrEq;
+
+TEST(AvogadroConstant, HasExpectedValue) {
+    // N_A = 6.02214076e23 mol^(-1)
+
+    // Test approximate value (guard against powers-of-10 type errors).
+    constexpr auto defining_units = inverse(moles) * pow<23>(mag<10>());
+    constexpr auto val = defining_units(6.02214076);
+    constexpr auto err = defining_units(0.00000001);
+    EXPECT_THAT(AVOGADRO_CONSTANT.as<double>(inverse(moles)), IsNear(val, err));
+
+    // Test exact value.
+    EXPECT_THAT(AVOGADRO_CONSTANT.in<int>(defining_units / pow<8>(mag<10>())),
+                SameTypeAndValue(602'214'076));
+}
+
+TEST(AvogadroConstant, HasExpectedLabel) {
+    EXPECT_THAT(unit_label(AVOGADRO_CONSTANT), StrEq("N_A"));
+}
+
+}  // namespace
+}  // namespace au
diff --git a/au/code/au/constants/test/boltzmann_constant_test.cc b/au/code/au/constants/test/boltzmann_constant_test.cc
new file mode 100644
index 00000000..2b411ec9
--- /dev/null
+++ b/au/code/au/constants/test/boltzmann_constant_test.cc
@@ -0,0 +1,48 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constants/boltzmann_constant.hh"
+
+#include "au/testing.hh"
+#include "au/units/joules.hh"
+#include "au/units/kelvins.hh"
+#include "gtest/gtest.h"
+
+namespace au {
+namespace {
+
+using symbols::J;
+using symbols::K;
+using ::testing::StrEq;
+
+TEST(BoltzmannConstant, HasExpectedValue) {
+    // k_B = 1.380649e-23 J/K
+
+    // Test approximate value (guard against powers-of-10 type errors).
+    constexpr auto defining_units = (joules / kelvin) * pow<-23>(mag<10>());
+    constexpr auto val = defining_units(1.380649);
+    constexpr auto err = defining_units(0.000001);
+    EXPECT_THAT(BOLTZMANN_CONSTANT.as<double>(J / K), IsNear(val, err));
+
+    // Test exact value.
+    EXPECT_THAT(BOLTZMANN_CONSTANT.in<int>(defining_units / pow<6>(mag<10>())),
+                SameTypeAndValue(1'380'649));
+}
+
+TEST(BoltzmannConstant, HasExpectedLabel) {
+    EXPECT_THAT(unit_label(BOLTZMANN_CONSTANT), StrEq("k_B"));
+}
+
+}  // namespace
+}  // namespace au
diff --git a/au/code/au/constants/test/cesium_hyperfine_transition_frequency_test.cc b/au/code/au/constants/test/cesium_hyperfine_transition_frequency_test.cc
new file mode 100644
index 00000000..ac94ce72
--- /dev/null
+++ b/au/code/au/constants/test/cesium_hyperfine_transition_frequency_test.cc
@@ -0,0 +1,39 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constants/cesium_hyperfine_transition_frequency.hh"
+
+#include "au/testing.hh"
+#include "au/units/hertz.hh"
+#include "gtest/gtest.h"
+
+namespace au {
+namespace {
+
+using ::testing::StrEq;
+
+TEST(CesiumHyperfineTransitionFrequency, HasExpectedValue) {
+    // Delta_nu_Cs = 9'192'631'770 Hz
+
+    // Test exact value.
+    EXPECT_THAT(CESIUM_HYPERFINE_TRANSITION_FREQUENCY.in<uint64_t>(hertz),
+                SameTypeAndValue(uint64_t{9'192'631'770}));
+}
+
+TEST(CesiumHyperfineTransitionFrequency, HasExpectedLabel) {
+    EXPECT_THAT(unit_label(CESIUM_HYPERFINE_TRANSITION_FREQUENCY), StrEq("Delta_nu_Cs"));
+}
+
+}  // namespace
+}  // namespace au
diff --git a/au/code/au/constants/test/elementary_charge_test.cc b/au/code/au/constants/test/elementary_charge_test.cc
new file mode 100644
index 00000000..e146d374
--- /dev/null
+++ b/au/code/au/constants/test/elementary_charge_test.cc
@@ -0,0 +1,44 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constants/elementary_charge.hh"
+
+#include "au/testing.hh"
+#include "au/units/coulombs.hh"
+#include "gtest/gtest.h"
+
+namespace au {
+namespace {
+
+using symbols::C;
+using ::testing::StrEq;
+
+TEST(ElementaryCharge, HasExpectedValue) {
+    // e = 1.602176634e-19 C
+
+    // Test approximate value (guard against powers-of-10 type errors).
+    constexpr auto defining_units = coulombs * pow<-19>(mag<10>());
+    constexpr auto val = defining_units(1.602176634);
+    constexpr auto err = defining_units(0.000000001);
+    EXPECT_THAT(ELEMENTARY_CHARGE.as<double>(C), IsNear(val, err));
+
+    // Test exact value.
+    EXPECT_THAT(ELEMENTARY_CHARGE.in<int>(defining_units / pow<9>(mag<10>())),
+                SameTypeAndValue(1'602'176'634));
+}
+
+TEST(PlanckConstant, HasExpectedLabel) { EXPECT_THAT(unit_label(ELEMENTARY_CHARGE), StrEq("e")); }
+
+}  // namespace
+}  // namespace au
diff --git a/au/code/au/constants/test/luminous_efficacy_540_terahertz_test.cc b/au/code/au/constants/test/luminous_efficacy_540_terahertz_test.cc
new file mode 100644
index 00000000..bba9f2f3
--- /dev/null
+++ b/au/code/au/constants/test/luminous_efficacy_540_terahertz_test.cc
@@ -0,0 +1,41 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constants/luminous_efficacy_540_terahertz.hh"
+
+#include "au/testing.hh"
+#include "au/units/lumens.hh"
+#include "au/units/watts.hh"
+#include "gtest/gtest.h"
+
+namespace au {
+namespace {
+
+using symbols::lm;
+using symbols::W;
+using ::testing::StrEq;
+
+TEST(LuminousEfficacy540Terahertz, HasExpectedValue) {
+    // K_cd = 683 lm/W
+
+    // Test exact value.
+    EXPECT_THAT(LUMINOUS_EFFICACY_540_TERAHERTZ.in<int>(lm / W), SameTypeAndValue(683));
+}
+
+TEST(LuminousEfficacy540Terahertz, HasExpectedLabel) {
+    EXPECT_THAT(unit_label(LUMINOUS_EFFICACY_540_TERAHERTZ), StrEq("K_cd"));
+}
+
+}  // namespace
+}  // namespace au
diff --git a/au/code/au/constants/test/planck_constant_test.cc b/au/code/au/constants/test/planck_constant_test.cc
new file mode 100644
index 00000000..fdbe601c
--- /dev/null
+++ b/au/code/au/constants/test/planck_constant_test.cc
@@ -0,0 +1,46 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constants/planck_constant.hh"
+
+#include "au/testing.hh"
+#include "au/units/joules.hh"
+#include "au/units/seconds.hh"
+#include "gtest/gtest.h"
+
+namespace au {
+namespace {
+
+using symbols::J;
+using symbols::s;
+using ::testing::StrEq;
+
+TEST(PlanckConstant, HasExpectedValue) {
+    // h = 6.62607015e-34 J s = 662607015e-41 J s
+
+    // Test approximate value (guard against powers-of-10 type errors).
+    constexpr auto defining_units = joule * seconds * pow<-34>(mag<10>());
+    constexpr auto val = defining_units(6.62607015);
+    constexpr auto err = defining_units(0.00000001);
+    EXPECT_THAT(PLANCK_CONSTANT.as<double>(J * s), IsNear(val, err));
+
+    // Test exact value.
+    EXPECT_THAT(PLANCK_CONSTANT.in<int>(defining_units / pow<8>(mag<10>())),
+                SameTypeAndValue(662'607'015));
+}
+
+TEST(PlanckConstant, HasExpectedLabel) { EXPECT_THAT(unit_label(PLANCK_CONSTANT), StrEq("h")); }
+
+}  // namespace
+}  // namespace au
diff --git a/au/code/au/constants/test/reduced_planck_constant_test.cc b/au/code/au/constants/test/reduced_planck_constant_test.cc
new file mode 100644
index 00000000..183262d3
--- /dev/null
+++ b/au/code/au/constants/test/reduced_planck_constant_test.cc
@@ -0,0 +1,55 @@
+// Copyright 2024 Aurora Operations, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "au/constants/reduced_planck_constant.hh"
+
+#include "au/constants/planck_constant.hh"
+#include "au/testing.hh"
+#include "au/units/joules.hh"
+#include "au/units/seconds.hh"
+#include "gtest/gtest.h"
+
+namespace au {
+namespace {
+
+using symbols::J;
+using symbols::s;
+using ::testing::StrEq;
+
+TEST(ReducedPlanckConstant, HasExpectedValue) {
+    // h_bar = (6.62607015e-34 / (2 pi)) J s ~= 1.054571817e-34 J s
+
+    // Test approximate value (guard against powers-of-10 type errors).
+    constexpr auto defining_units = joule * seconds * pow<-34>(mag<10>());
+    constexpr auto val = defining_units(1.054571817);
+    constexpr auto err = defining_units(0.000000001);
+    EXPECT_THAT(REDUCED_PLANCK_CONSTANT.as<double>(J * s), IsNear(val, err));
+}
+
+TEST(ReducedPlanckConstant, ExactlyPlanckConstantDividedByTwoPi) {
+    constexpr auto ratio = (REDUCED_PLANCK_CONSTANT * mag<2>() * Magnitude<Pi>{}) / PLANCK_CONSTANT;
+
+    // We know that `.as<int>()` could not succeed unless the result were exactly expressible with
+    // `int` as a rep, and we know that the comparison to `1` would not compile unless all
+    // dimensions had cancelled out.
+    constexpr int result = ratio.as<int>();
+    EXPECT_EQ(result, 1);
+}
+
+TEST(ReducedPlanckConstant, HasExpectedLabel) {
+    EXPECT_THAT(unit_label(REDUCED_PLANCK_CONSTANT), StrEq("h_bar"));
+}
+
+}  // namespace
+}  // namespace au