Going to a jewelry store to have your finger sized would probably be the easiest and most accurate way. But by any chance you cant get to a jewelry store, you could cut a strip of paper 1/2 wide and about 5 inches long. Wrap the paper around your finger and be sure to mark where it overlaps. Remove it from your finger and with a ruler, measure the marked end with a ruler (metric side of greater accuracy). Whichever way you get it done, just remember that if you have any doubt at all about the size, its ALWAYS better to go a little bit larger rather than smaller.