Convert a ring size between countries

The job

Buying a ring from an overseas shop, or reading a size chart that uses a different system. Japan counts in 号, the US in numbers, the UK in letters, and the EU uses the inner circumference in millimetres. With a ruler you can also start from a ring you already own.

How to do it

Choose what you know — a size in one system, or the inside diameter or circumference in mm — and enter the value. The tool shows the equivalents in all systems at once.

• JP sizes use the standard formula: inner circumference ≈ π × (号 + 38) / 3 mm.
• To measure an existing ring, lay a ruler across the inside and enter the diameter.

Worked example

A JP 13 ring converts to about US 6.5, UK M, EU 52, with an inner diameter of roughly 17 mm. Entering 16.7 mm as a diameter snaps to the nearest standard row instead — around JP 13 / US 6.5.

Limits

US, UK, EU, and measured values snap to the nearest standard row, so they are approximations. Real rings vary by maker, and wide bands fit tighter. The chart covers JP 1–36 (about 14–23.5 mm inside diameter); confirm the fit in person before buying.