Ring Size by Circumference Converter
Turn a finger circumference or diameter in millimetres into US, ISO/EU and UK ring sizes — measure once, convert everywhere.
How it works
circumference = π × diameter · US size = (circumference mm − 36.537) ÷ 2.5535 · ISO/EU size = inside circumference in mm
Ring sizes are all derived from one physical number: the inside circumference of the band. The ISO (and most of Europe) simply uses that circumference in millimetres as the size. The US system is a linear scale where each full size adds about 2.55 mm of circumference (0.8 mm of diameter). Measuring the finger — or an existing ring — in millimetres lets you convert to any national scale without a printed sizer.
Worked example
A 54 mm inside circumference gives a diameter of 54 ÷ π = 17.19 mm. US size = (54 − 36.537) ÷ 2.5535 = 6.84 → 7. ISO/EU size is simply 54, and the UK equivalent is around O.
Frequently asked questions
Should I size for the knuckle or the base of the finger?
The ring has to slide over the knuckle but sit at the base, so the ideal size is a compromise: snug at the base but able to pass the knuckle with light resistance. If the knuckle is much larger, size up slightly.
When is the best time to measure?
Late in the day and at a normal temperature. Fingers shrink in the cold and swell in heat or after salt and exercise, easily by half a size, so avoid measuring first thing in the morning or right after a workout.
How do I measure an existing ring?
Measure the inside diameter across the widest point in millimetres and choose the diameter mode, or wrap a strip of paper around the inside and measure its length for the circumference. Either feeds this converter directly.
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This calculator is for educational purposes. Double-check important results before acting on them.