How this calculator works
Every ring size system describes the same circle, so the reliable route between them is the inside diameter or the inside circumference. The circumference is the diameter multiplied by pi. The US scale starts at 11.63 mm inside diameter for size 0 and adds about 0.81 mm for each size, which is why size 7 is about 17.3 mm. UK letters start at A with about 11.95 mm and run in steps of about 0.4 mm, with half letters in between. European sizes are close to the circumference in millimetres, German sizes are the inside diameter in millimetres, and Japanese sizes count from the circumference minus about 40.
Formula
circumference = diameter x pi. US size = (diameter - 11.63) / 0.8128. UK letter = the letter covering the diameter from A at about 11.95 mm in 0.398 mm steps. European size = circumference in mm, rounded. German size = inside diameter in mm. Japanese size = circumference in mm - 40.4, rounded.
Worked example
A ring with a 17.3 mm inside diameter has a circumference of 54.4 mm. That is (17.3 - 11.63) / 0.8128 = 6.98, so US size 7. It falls at UK N 1/2, European 54, German 17.25 and Japanese 14.
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Frequently asked questions
How do I measure a ring that fits?
Measure the inside diameter across the widest point with a ruler or callipers. Do not measure the outside, which includes the metal thickness. If you measure a ring that is not perfectly round, use the average of the widest and narrowest inside measurements.
What if I am between sizes?
Ring sizes are usually sold in half or quarter steps. Choose the larger size for a wide band, because a wide band sits further up the finger, and the smaller size for a narrow band. Fingers also change size with temperature and time of day.
Why do the European and German figures differ?
They measure different parts of the same circle. European sizes are close to the circumference in millimetres, while German sizes use the inside diameter in millimetres. A 54.4 mm circumference is European 54 and German 17.25.
Is a paper strip accurate enough?
It is a good starting point, but paper stretches and a loose strip reads large. Wrap it snugly, mark the overlap and measure the length. For an expensive ring, have a jeweller measure you with a set of ring sizers.