AN is very easy to figure out.............everything is based on 16ths...............so 4/16= 1/4", 8/16=1/2" and all odd numbers are what they are already like 3/16 and 5/16............and when you get to bigger sizes like #20AN ,again you take 20over 16 and get 1-1/4".
This is how I had figured it out long time ago in a galaxy far far away.