Ring Size Conversion Calculator

What each national scale actually measures

All four scales describe one physical quantity — the inside circumference — and differ only in how they label it. The US scale is linear in circumference: 36.537 mm at size zero, plus 2.5535 mm per size. Europe takes the ISO circumference and drops the leading forty, so 54.5 mm becomes 14.5. Japan works from diameter instead, starting at 13 mm for size 1 and adding a third of a millimetre per size.

The UK is the odd one out because it is alphabetical. It advances in half-letter steps at four steps to each US size, which is why US half sizes land on whole letters: US 6 is L½, US 6.5 is M½, US 7 is N½.

The practical consequence is that no two scales share a number for the same ring, and two of them use numbers in overlapping ranges. A European 14 and a Japanese 13 are close in value and nearly two sizes apart in reality.

A worked example

A US 7 has a circumference of 36.537 plus 2.5535 times 7, which is 54.41 mm. Subtracting forty gives a European 14.4, usually sold as 14.5. The diameter is 54.41 divided by pi, or 17.32 mm, which puts it at a Japanese 13.

In the UK the same ring is four half-steps above US 6's L½, so N½. Anyone quoting that ring as 'a 7' to a British jeweller would be understood as asking for a size G — nearly three sizes small, and a ring that would not pass the knuckle.

Where the answer misleads

Retailers are inconsistent about the European scale. Some quote the ISO circumference in full, so the same ring appears as a 54 rather than a 14. If a European size looks like a two-digit number in the fifties, it is a circumference; if it is in the teens, it has had the forty removed.

Half sizes are not stocked everywhere. Much of continental Europe sells in whole millimetres of circumference, which is a finer gradation than US half sizes, while some UK ranges skip the half letters entirely. Converting to an exact figure the shop does not make is a common source of a ring that is a quarter size out.

None of this accounts for band width or knuckle size. The conversion is exact; the fit is not. For a wide band, add a quarter to a half size after converting.