diff --git a/src/core/include/mp-units/bits/fmt.h b/src/core/include/mp-units/bits/fmt.h
index 8ffb7284..bd558724 100644
--- a/src/core/include/mp-units/bits/fmt.h
+++ b/src/core/include/mp-units/bits/fmt.h
@@ -36,6 +36,21 @@
 #include <concepts>
 #include <limits>
 #include <string_view>
+
+#if MP_UNITS_USE_FMTLIB
+MP_UNITS_DIAGNOSTIC_PUSH
+MP_UNITS_DIAGNOSTIC_IGNORE_UNREACHABLE
+MP_UNITS_DIAGNOSTIC_IGNORE_SHADOW
+#include <fmt/format.h>
+MP_UNITS_DIAGNOSTIC_POP
+#else
+#include <format>
+#endif
+
+#ifndef MP_UNITS_IN_MODULE_INTERFACE
+#include <cstdint>
+#endif
+
 #endif
 
 // most of the below code is based on/copied from fmtlib
diff --git a/src/core/include/mp-units/compat_macros.h b/src/core/include/mp-units/compat_macros.h
index a231bfd8..b379350f 100644
--- a/src/core/include/mp-units/compat_macros.h
+++ b/src/core/include/mp-units/compat_macros.h
@@ -52,12 +52,6 @@
 
 #if MP_UNITS_USE_FMTLIB
 
-MP_UNITS_DIAGNOSTIC_PUSH
-MP_UNITS_DIAGNOSTIC_IGNORE_UNREACHABLE
-MP_UNITS_DIAGNOSTIC_IGNORE_SHADOW
-#include <fmt/format.h>
-MP_UNITS_DIAGNOSTIC_POP
-
 #define MP_UNITS_STD_FMT fmt
 #define MP_UNITS_FMT_LOCALE(loc) (loc).template get<std::locale>()
 #define MP_UNITS_FMT_TO_ARG_ID(arg) (arg)
@@ -83,8 +77,6 @@ MP_UNITS_DIAGNOSTIC_POP
 #error "std::formatting facility not supported"
 #endif
 
-#include <format>
-
 #define MP_UNITS_STD_FMT std
 #define MP_UNITS_FMT_LOCALE(loc) loc
 #define MP_UNITS_FMT_TO_ARG_ID(arg) static_cast<std::size_t>(arg)