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

inside circumference (in mm) inside diameter (in mm)
Size 5 48.8 15.4
Size 6 51.8 16.4
Size 7 54.5 17.3
Size 8 57.7 18.3
Size 9 59.5 18.9