Ring Size Converter

The Ring Size Converter cross-references a ring size across the systems used around the world, so a size bought in one country can be matched in another.

Enter a US/Canada number, an EU/ISO size, a Japanese size, or a physical measurement of the band — its inner diameter or inner circumference in millimetres — and it returns the equivalent in every system at once, including the UK/Australia letter size.

Every conversion is anchored to the ring’s inner circumference, which is what sizing physically depends on, so the results stay mutually consistent rather than drifting between separate lookup tables.

The EU/ISO number is simply that inner circumference in millimetres (the ISO 8653 basis), the US scale follows the standard linear relationship of roughly 0.81 mm per half size, the Japanese scale tracks the common JP = US × 2 − 0.5 mapping, and the UK letters step through the alphabet in half-letter increments.

The most reliable way to size a ring is to measure one that already fits the intended finger: wrap a strip of paper around the inside of the band, or measure its inner diameter with a ruler, and enter that millimetre value here.

Measuring the finger directly tends to read large because the knuckle pushes the tape outward.

Bear in mind that manufacturers and individual jewellers vary by up to half a size, so for an engagement or wedding band confirm against the seller’s own chart.

Everything is calculated locally in your browser, with nothing uploaded, and it keeps working offline.