diff --git a/cpdftruetype.ml b/cpdftruetype.ml
index e1abb42..8461cf0 100644
--- a/cpdftruetype.ml
+++ b/cpdftruetype.ml
@@ -55,7 +55,7 @@ let discard_bytes b n =
   for x = 1 to n do ignore (getval_31 b 8) done
 
 let pdf_unit unitsPerEm x =
-  int_of_float (float_of_int x *. 1000. /. float_of_int unitsPerEm)
+  int_of_float (float_of_int x *. 1000. /. float_of_int unitsPerEm +. 0.5)
 
 let string_of_tag t =
   Printf.sprintf "%c%c%c%c"