Some slight annomolies with Kevs sizing.
1/2" =12.7mm approx 13mm
9/16" =14.28mm approx 14mm
5/8" =15.875mm approx 16mm
11/16" = 17.46mm Nearer 17 than 18
3/4" = 19mm or as near as makes no difference.
Soory Kev just reread your post and it wasn't conversion but the size hierarchy