How do you figure out what size ring you wear without a ring sizer?

like is there a way in inches or millimeters per each size?How do you figure out what size ring you wear without a ring sizer?
http://www.bluenile.com/find-ring-size





http://www.atlantisring.com/free_ring_si鈥?/a>





these are two different ones you can print--the first one you use a ring that fits properly, the second one you use a piece of string wrapped around your finger