I get this question a lot, especially during holidays when people are purchasing gifts for loved ones.  Here is my suggestion on finding out your (or someone else's) ring size.

1.  DO NOT GUESS!  To resize a ring is a cumbersome and expensive endeavor.  

2.  Visit a local jewelry store!  They have ring sizers and would be happy to size your finger, whether you buy from them or not.

3.  Use this website!  I have found this to be a credible method of getting your proper ring size.  Carefully follow the directions -  Ring Sizing