Chainmaille Ring Sizer Tool (Inside Diameter)

While working with various ring sizes, I've often picked up a ring and wondered what size it is. This solves the problem without the need for calipers or a ruler. Measurements are in inches and millimetres, so conversion can also be done. This particular ring sizer measures inside diameter (ID).

You can find the one that measures outside diameter here:
http://www.thingiverse.com/thing:835401