diff --git a/libavutil/base64.h b/libavutil/base64.h
index 103860eff0..477128afab 100644
--- a/libavutil/base64.h
+++ b/libavutil/base64.h
@@ -46,4 +46,9 @@ int av_base64_decode(uint8_t *out, const char *in, int out_size);
  */
 char *av_base64_encode(char *out, int out_size, const uint8_t *in, int in_size);
 
+/**
+ * Calculate the output size needed to base64-encode x bytes.
+ */
+#define AV_BASE64_SIZE(x)  (((x)+2) / 3 * 4 + 1)
+
 #endif /* AVUTIL_BASE64_H */