S (15-16 mm) M (17 mm) L (18-19 mm) Products are handmade durable glass. It is not a mold. :) How to Learn Ring Size? :) * You can easily find out your size by measuring the circumference of your finger using the string. You can complete the circumference of your finger with the string and note the length with a meter. * You can also measure the inner diameter of another ring you have used before, at its widest point.