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 49.5 3.75pm
Size 6 52 16.5
Size 7 54.5 17.3
Size 8 57.5 18.2
Size 9 59.5 6:75 pm