Measure a ring’s inside diameter or your finger’s circumference in millimetres and get the closest US, UK/AU and EU ring sizes.
How ring sizing works
Every ring-size system is just a label on the same physical measurement: the inside circumference in millimetres. The US runs on numbers, the UK and Australia on letters, and most of Europe uses the circumference itself. Measure once — either the inside diameter of a ring that fits, or a snug string around the finger — and this calculator converts to all three, flagging anything outside the standard range instead of forcing a bad match.
How to use it
- Pick what you measured: inside diameter of an existing ring, or finger circumference.
- Enter the millimetres.
- Read the closest US, UK/AU and EU sizes.
The conversion
A worked example
A ring with a 17.3 mm inside diameter has a circumference of 54.3 mm — that is US 7, UK N, EU 54, the single most common women’s ring size. Measure in the evening (fingers swell during the day) and prefer a ring that already fits the intended finger over a string measurement when you can — it is the more reliable of the two.