About this tool
Convert any ring size between the US/Canada numeric scale, UK/Australia letter sizes, the EU/ISO scale (the ring's inner circumference in millimetres), Japanese sizes, and the raw inner diameter and circumference in millimetres. Enter the one value you know — a US 7, a UK N½, an EU 54 or a measured 17.3 mm diameter — and every equivalent appears instantly, snapped to the nearest standard half size.
All systems describe the same thing: the inside measurement of the band. The ISO 8653 standard simply uses the inner circumference in millimetres, which is why an EU 54 has a circumference of about 54 mm. US sizes step by roughly 0.81 mm of diameter per size, UK sizes step one letter at a time, and Japanese sizes step about 1 mm of circumference. To measure at home, wrap a thin strip of paper snugly around the finger, mark where it overlaps, and measure the length in millimetres — that is the circumference, which you can enter directly as the EU/ISO value.
The conversion runs entirely in your browser from a built-in ISO-style table; nothing is uploaded and the tool works offline. Sizes are standardised, but ring width matters too — wide bands fit tighter, so consider going up half a size for bands over 6 mm.