50cm = 19 and 5/8"
25cm =about 9 and 7/8"
15cm = about 5 and 7/8"
12cm = 4 and 3/4 inches
5cm = just a whisker under 2"
25mm = 1"
6mm = pretty close to 1/4"
I hope this helps a bit, are you able to get rulers from your stationery store that has centimetres and milimetres? Then you could just put it on your inch ruler and convert them that way.