diff --git a/sys/include/base64.h b/sys/include/base64.h
index c34534059f..4509c9d28e 100644
--- a/sys/include/base64.h
+++ b/sys/include/base64.h
@@ -41,7 +41,7 @@ extern "C" {
  */
 static inline size_t base64_estimate_decode_size(size_t base64_in_size)
 {
-    return ((base64_in_size / 4) * 3);
+    return (((base64_in_size + 3) / 4) * 3);
 }
 
 /**