diff --git a/Core/include/Acts/Definitions/Units.hpp b/Core/include/Acts/Definitions/Units.hpp
index 4f5bd779b3a..5a247e5eccf 100644
--- a/Core/include/Acts/Definitions/Units.hpp
+++ b/Core/include/Acts/Definitions/Units.hpp
@@ -202,10 +202,10 @@ constexpr double mol = 1.0;
 namespace UnitLiterals {
 // define user literal functions for the given unit constant
 #define ACTS_DEFINE_UNIT_LITERAL(name)                        \
-  consteval double operator"" _##name(long double x) {        \
+  inline consteval double operator"" _##name(long double x) {        \
     return ::Acts::UnitConstants::name * x;                   \
   }                                                           \
-  consteval double operator"" _##name(unsigned long long x) { \
+  inline consteval double operator"" _##name(unsigned long long x) { \
     return ::Acts::UnitConstants::name * x;                   \
   }
 ACTS_DEFINE_UNIT_LITERAL(fm)