Old School Ring Sizing:
Take a piece of string and wrap it around your finger and where the end of the string meets the string that is your ring size.
Use a pen to mark this spot or cut with scissors.
Measure millimeters with ruler, this is your ring circumference.
Divide the circumference by 3.14 and you get your ring size diameter.
For the best fit possible have a professional jeweler size the finger that you plan on wearing your ring.