Tip: take a string to measure the circumference of your finger or measure the diameter (of the inside) of another ring.

inner circumference (in mm) inside diameter (in mm)
Size 5 48.8 15.4
Size 6 51.7 16.3
Size 7 54 17.2
Size 8 57.3 18.3
Size 9 59.4 18.8