diff --git a/std/utf.d b/std/utf.d
index 4d4f7d77407..55251ad149e 100644
--- a/std/utf.d
+++ b/std/utf.d
@@ -138,7 +138,8 @@ unittest
     $(D index) defaults to $(D 0) if none is passed.
 
     Returns:
-        The number of bytes in the UTF-8 sequence, a value between 1 and 4.
+        The number of bytes in the UTF-8 sequence, a value between 1 and 4
+        (as per $(WEB tools.ietf.org/html/rfc3629#section-3, RFC 3629$(COMMA) section 3)).
 
     Throws:
         Throws a $(D UTFException) if $(D str[index]) indicates an invalid