diff --git a/library/base64.c b/library/base64.c
index 4170610..3eb9e7c 100644
--- a/library/base64.c
+++ b/library/base64.c
@@ -17,6 +17,8 @@
  *  limitations under the License.
  */
 
+#include <limits.h>
+
 #include "common.h"
 
 #if defined(MBEDTLS_BASE64_C)
@@ -31,8 +33,6 @@
 #include "mbedtls/platform.h"
 #endif /* MBEDTLS_SELF_TEST */
 
-#define BASE64_SIZE_T_MAX   ((size_t) -1)   /* SIZE_T_MAX is not standard */
-
 /*
  * Encode a buffer into base64 format
  */
@@ -50,8 +50,8 @@
 
     n = slen / 3 + (slen % 3 != 0);
 
-    if (n > (BASE64_SIZE_T_MAX - 1) / 4) {
-        *olen = BASE64_SIZE_T_MAX;
+    if (n > (SIZE_MAX - 1) / 4) {
+        *olen = SIZE_MAX;
         return MBEDTLS_ERR_BASE64_BUFFER_TOO_SMALL;
     }
 
diff --git a/library/bignum.c b/library/bignum.c
index e686a1b..36effaf 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -54,8 +54,6 @@
 #define MPI_VALIDATE(cond)                                           \
     MBEDTLS_INTERNAL_VALIDATE(cond)
 
-#define MPI_SIZE_T_MAX  ((size_t) -1)   /* SIZE_T_MAX is not standard */
-
 /* Implementation that should never be optimized out by the compiler */
 static void mbedtls_mpi_zeroize(mbedtls_mpi_uint *v, size_t n)
 {
@@ -416,7 +414,7 @@
     slen = strlen(s);
 
     if (radix == 16) {
-        if (slen > MPI_SIZE_T_MAX >> 2) {
+        if (slen > SIZE_MAX >> 2) {
             return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
         }