Ring Size Converter

US and Canadian numeric sizes, UK and Australian letters, EU and ISO 8653 circumference sizes, Japanese numbers and inside diameter in millimetres — all converted through the quantity that actually fits the finger, the inside circumference in millimetres. With how to measure, and the warnings that matter.

One circumference, four conventions

Every half size, computed from the published definitions

How to measure, and what goes wrong

Ring sizes are conventions on one measurement

There is exactly one number that decides whether a ring fits, and it is the distance round the inside of the band. ISO 8653 takes the honest route and makes that number the size: a size 54 ring has an inside circumference of 54 mm, and that is the whole definition. Every other system in use is a convention laid over the same quantity. The American scale counts 0.032 of an inch of inside diameter per whole size. The British and Australian scale runs through the alphabet, and since the 1987 revision one letter division is 1.25 mm of circumference with size C at 40 mm. The Japanese scale adds a third of a millimetre of diameter per size from 13 mm at size 1. Convert through the circumference and all four agree; convert chart-to-chart and they drift.

How to measure, properly. Wrap a strip of paper round the base of the finger, mark where the end meets, and measure the marked length in millimetres. That is the circumference, no conversion required. Check the strip will slide over the knuckle — for a large knuckle the knuckle decides the size, not the base of the finger. Do it two or three times and take the largest reading, because paper stretches. If you are copying an existing ring, measure across the inside at the widest point, in millimetres, and use the diameter mode. Printed paper sizers are the least reliable method on this list, because printers rescale silently; if you use one, check its printed ruler against a real one first.

The warnings that actually cost people money. Fingers change size. Heat, salt, exercise, pregnancy, air travel and the time of day all move them, and a full size between a cold morning and a warm evening is ordinary rather than remarkable — so measure at the end of a normal day, at a normal temperature, and not straight after the gym. A wide band needs a larger size than a narrow one: a 2 mm band and an 8 mm band that measure the same circumference do not feel the same, because the wide one sits across more of the finger and cannot flex, and jewellers routinely fit wide bands a quarter to a half size up. And not every ring can be resized. A plain metal band usually can. A full eternity ring, a channel or pavé setting that runs the whole way round, a tension setting, titanium, tungsten carbide and ceramic usually cannot, or not without remaking the ring. If the ring is a surprise and the setting is one of those, buy the setting last.

On the Japanese scale, a caveat. The rule used here — a third of a millimetre of diameter per size — reproduces a published JIS table to within 0.2 mm across its whole range. A separate published cross-reference table maps Japanese sizes onto US sizes up to two thirds of a US size away from that. Both are real published sources and they disagree, so a Japan conversion is worth confirming with the seller rather than trusting to the last digit. For footwear, where the same problem of conventions over a measurement appears in a worse form, see the shoe size converter. The figures behind this conversion come from a published table, not a definition, so a different source may give slightly different numbers. The reference below is the one used here.

Frequently asked questions

Related calculators

References

1. ISO 8653:2016, Jewellery — Ring sizes — Definition, measurement and designation. Cited by number; the standard is copyrighted and no table from it is reproduced. Its definition is the one this page converts through: the size is the inside circumference of the ring in millimetres.
2. Wikipedia, Ring size (read 26 September 2026): US and Canadian whole sizes are 0.032 in (0.81 mm) of inside diameter apart, equivalently 0.1005 in (2.55 mm) of circumference; the 1987 British revision makes one letter division 1.25 mm of circumference with size C at 40 mm.
3. Maverick Store, リングサイズ表（JIS規格）, maverick-stores.com/notes/size/ (read 26 September 2026) — the JIS table, sizes 1 to 28: size 1 is 40.8 mm of inside circumference and size 28 is 69.0 mm, which is a step of exactly 1/3 mm of diameter per size.