International ring size converter

Six systems, one measurement

Every ring size system measures the same thing: the inside of the band. The US and Canadian scale starts at 11.63 mm inside diameter for size 0 and adds 0.8128 mm (0.032") per whole size, so each quarter size is about 0.2 mm. There is no ASTM or ANSI standard behind it; it is a trade scale that ring mandrels and sizing gauges follow.

ISO 8653, used across continental Europe, simply names a ring by its inside circumference in millimetres, so EU 54 is a 54 mm loop. Swiss, Italian and Spanish sizes are that number minus 40. The UK, Ireland and Australia use letters; since BS 6820 (1987) each letter is 1.25 mm of circumference, with C at 40 mm, and half letters in between. Japan uses numbers 1 to 30 that step about a third of a millimetre of diameter.

US inside Ø (mm) = 11.63 + 0.8128 × size · circumference = π × Ø · EU = circumference rounded · Swiss = EU − 40

Where conversions go wrong

The systems step at different intervals, so a ring rarely sits exactly on a size in every system at once. A US quarter size is 0.64 mm of circumference, a UK half letter 0.625 mm and an ISO size 1 mm, so two charts can disagree by a step at the edges. This converter computes every column from the same circumference instead of copying printed tables, so the columns always agree with each other.

UK charts printed before 1987 followed an older scale based on 1/64" steps of diameter, and some still circulate with A at 37.8 mm instead of 37.5 mm. If a UK shop is very old-fashioned, its letters may run about a quarter size different from the metric BS 6820 scale used here.

Japan, China and Hong Kong

Japanese sizes follow a published table (JCS), from No. 1 at 13.0 mm inside to No. 30 at 22.7 mm, about a third of a millimetre per size. They are not a simple formula of the circumference, so this site uses the table itself.

Mainland China and Hong Kong sizes are often listed as the same as Japan’s, but published charts disagree, and we found no public standard to rely on. When buying from a Chinese or Hong Kong seller, ask for the inside diameter or circumference in millimetres and compare it with your measurement here.

German and Dutch sizes

Germany and the Netherlands traditionally name rings by inside diameter in millimetres, in half-millimetre steps, so a German 17 is a 17.0 mm ring and roughly a US 6½. The last column of the chart gives that diameter for each ISO size.