diff --git a/src/core/opamStd.ml b/src/core/opamStd.ml
index 9b31a42b8f9..013db09e45f 100644
--- a/src/core/opamStd.ml
+++ b/src/core/opamStd.ml
@@ -799,20 +799,20 @@ module OpamFormat = struct
   let visual_length_substring s ofs len =
     let rec aux acc i =
       if i >= len then acc
-      else match s.[i] with
-        | '\xc2'..'\xdf' -> aux (acc + (min 1 (len - i))) (i + 2)
-        | '\xe0'..'\xef' -> aux (acc + (min 2 (len - i))) (i + 3)
-        | '\xf0'..'\xf4' -> aux (acc + (min 3 (len - i))) (i + 4)
+      else match s.[ofs + i] with
+        | '\xc2'..'\xdf' -> aux (acc - min 1 (len - i)) (i + 2)
+        | '\xe0'..'\xef' -> aux (acc - min 2 (len - i)) (i + 3)
+        | '\xf0'..'\xf4' -> aux (acc - min 3 (len - i)) (i + 4)
         | '\027' ->
           (try
-             let j = String.index_from s (i+1) 'm' in
-             if j > ofs + len then acc + len - i else
-               aux (acc + j - i + 1) (j + 1)
+             let j = String.index_from s (ofs+i+1) 'm' - ofs in
+             if j > len then acc - (len - i) else
+               aux (acc - (j - i + 1)) (j + 1)
            with Not_found | Invalid_argument _ ->
-             acc + len - i)
+             acc - (len - i))
         | _ -> aux acc (i + 1)
     in
-    len - aux 0 ofs
+    aux len 0
 
   let visual_length s = visual_length_substring s 0 (String.length s)