diff --git a/include/ruby/internal/arithmetic/size_t.h b/include/ruby/internal/arithmetic/size_t.h
index 8341c80a35..0458f1f5f3 100644
--- a/include/ruby/internal/arithmetic/size_t.h
+++ b/include/ruby/internal/arithmetic/size_t.h
@@ -27,25 +27,30 @@
 #include "ruby/backward/2/long_long.h"
 
 #if SIZEOF_SIZE_T == SIZEOF_LONG_LONG
-# define SIZET2NUM  RB_ULL2NUM
-# define SSIZET2NUM RB_LL2NUM
+# define RB_SIZE2NUM RB_ULL2NUM
+# define RB_SSIZE2NUM RB_LL2NUM
 #elif SIZEOF_SIZE_T == SIZEOF_LONG
-# define SIZET2NUM  RB_ULONG2NUM
-# define SSIZET2NUM RB_LONG2NUM
+# define RB_SIZE2NUM RB_ULONG2NUM
+# define RB_SSIZE2NUM RB_LONG2NUM
 #else
-# define SIZET2NUM  RB_UINT2NUM
-# define SSIZET2NUM RB_INT2NUM
+# define RB_SIZE2NUM RB_UINT2NUM
+# define RB_SSIZE2NUM RB_INT2NUM
 #endif
 
 #if SIZEOF_SIZE_T == SIZEOF_LONG_LONG
-# define NUM2SIZET  RB_NUM2ULL
-# define NUM2SSIZET RB_NUM2LL
+# define RB_NUM2SIZE RB_NUM2ULL
+# define RB_NUM2SSIZE RB_NUM2LL
 #elif SIZEOF_SIZE_T == SIZEOF_LONG
-# define NUM2SIZET  RB_NUM2ULONG
-# define NUM2SSIZET RB_NUM2LONG
+# define RB_NUM2SIZE RB_NUM2ULONG
+# define RB_NUM2SSIZE RB_NUM2LONG
 #else
-# define NUM2SIZET  RB_NUM2UINT
-# define NUM2SSIZET RB_NUM2INT
+# define RB_NUM2SIZE RB_NUM2UINT
+# define RB_NUM2SSIZE RB_NUM2INT
 #endif
 
+#define NUM2SIZET RB_NUM2SIZE
+#define SIZET2NUM RB_SIZE2NUM
+#define NUM2SSIZET RB_NUM2SSIZE
+#define SSIZET2NUM RB_SSIZE2NUM
+
 #endif /* RBIMPL_ARITHMETIC_SIZE_T_H */