Ring Size Converter
Convert ring sizes between US, European and inside diameter or circumference.
Good to know. Fingers are biggest on warm evenings and smallest on cold mornings. Measure more than once.
Approximate. Real-world values vary.
How it works
In plain words
US sizes go up by 0.8128 mm of inside diameter per size, starting from 11.63 mm at size 0. The European size is the inside circumference in millimetres: π times the diameter.
The formula
diameter (mm) = 11.63 + 0.8128 × US size EU size = circumference = π × diameter
Worked example
A US size 7 has an inside diameter of 11.63 + 0.8128 × 7 = 17.3 mm and a circumference of 54.4 mm, which is European size 54.5.
Sources
- ISO 8653, Jewellery: ring sizes, definition, measurement and designation European ring sizes as the inside circumference in millimetres.
Sources checked September 30, 2026. 4 automated tests check its answers on every change.
About the ring size converter
Ring sizes are numbered differently around the world. The US and Canada use numbers from about 3 to 13 in quarter steps; most of Europe uses the inside circumference in millimetres (ISO 8653), so a size 54 ring measures 54 mm around. This converter translates between them, and from a measurement of your finger or of a ring that fits.
To measure a finger, wrap a strip of paper or a piece of string around the base of the finger, mark where it overlaps, and measure the length in millimetres: that is the circumference. To measure a ring that fits, measure straight across the inside, edge to edge, for the diameter.
Measure at the end of the day, when fingers are largest, and not when you are cold. The ring has to pass over the knuckle, so if your knuckle is large, a size between your finger and knuckle measurements is best. Wide bands feel tighter than thin ones; go up a quarter or half size for bands over 6 mm.
The UK, Ireland and Australia use letters (A to Z), and Japan its own numbers; ask the jeweller for their chart. Sizes can differ slightly between makers, so for an expensive ring a jeweller’s sizing set is the surest check.
Questions people ask
What is a US size 7 in European sizes?
About 54.5: a circumference of 54.4 mm and an inside diameter of 17.3 mm.
How do I measure my ring size at home?
Wrap paper or string around the base of your finger, mark the overlap, and measure it in millimetres. That circumference is your European size.
What is the most common ring size?
Around US 6 to 7 for women and 9 to 10 for men, but sizes vary widely.