The following ring size guide is perfect if you're looking purchase a ring online, and you are unsure of the exact size. If you have an existing ring that is a comfortable fit on your finger and doesn't slip off your knuckle, then that would be perfect to find your ring size.
How to measure ring size:
Step 1 - Print out the below ring size chart at 100% and make sure the line in the image measures exactly 135mm.
Step 2 - Place your ring so that the black circle is exactly touching the inside of your ring. This will give you your correct ring sizing.
If you hesitate between 2 sizes, choose the larger one.