This convenient ring sizer, manufactured in the UK, works like a belt. Push the end through the buckle to form a ring shape, then slip the ring sizer on the your finger (take care to measure the hand you are going to wear the ring on, as the fingers on our left and right hands differ in size). Adjust the belt for a comfortable fit checking that it just slips over the knuckle.

The letter just to the left of the arrow is your ring size.