diff --git a/example/kalman_filter/kalman.h b/example/kalman_filter/kalman.h
index bcf19646..5efd0df2 100644
--- a/example/kalman_filter/kalman.h
+++ b/example/kalman_filter/kalman.h
@@ -24,6 +24,7 @@
 
 #include <mp-units/compat_macros.h>
 #include <algorithm>
+#include <locale>
 #include <tuple>
 #ifdef MP_UNITS_MODULES
 import mp_units;
diff --git a/src/core/include/mp-units/bits/core_gmf.h b/src/core/include/mp-units/bits/core_gmf.h
index dfb93961..a64e46b1 100644
--- a/src/core/include/mp-units/bits/core_gmf.h
+++ b/src/core/include/mp-units/bits/core_gmf.h
@@ -35,6 +35,7 @@
 #include <initializer_list>
 #include <iterator>
 #include <limits>
+#include <locale>
 #include <numbers>
 #include <numeric>
 #include <optional>
diff --git a/src/core/include/mp-units/format.h b/src/core/include/mp-units/format.h
index 88616c1f..0400b492 100644
--- a/src/core/include/mp-units/format.h
+++ b/src/core/include/mp-units/format.h
@@ -32,6 +32,10 @@
 #include <mp-units/framework/quantity.h>
 #include <mp-units/framework/unit.h>
 
+#ifndef MP_UNITS_IN_MODULE_INTERFACE
+#include <locale>
+#endif
+
 namespace mp_units::detail {
 
 template<typename Char>