Ring Size Converter
Convert ring sizes between US/Canada numbers, UK/Australia letters and the EU/ISO standard, with the inside diameter and circumference in millimetres. Ring sizing comes down to one measurement — the inside diameter of the band — which every system encodes differently. Enter a size in any system to see all the others, so you can order confidently from a shop that uses a different scale.
How to use this tool
- Enter a ring size — a US number, a UK letter, or a measured diameter or circumference in millimetres.
- Select which system your value is in.
- Read the equivalents in every system, with the exact inside diameter and circumference.
- To find your size, wrap paper around your finger and measure the circumference, then enter it here.
- Measure at the end of the day when fingers are largest, for a comfortable fit.
The formula
Ring size is defined by the band’s inside diameter. Circumference follows directly, and each regional scale is a labelling of that diameter. The EU/ISO standard simply is the circumference in millimetres.
circumference = π × diameter
ISO/EU size = circumference in mm
US and UK sizes map to diameter via the reference table- inside diameter
- The width across the inside of the band, in mm
- π
- Pi, ≈ 3.14159, linking diameter and circumference
US sizes step by about 0.4 mm of diameter per half size. Wide bands fit more snugly, so size up a little for bands over about 6 mm wide.
Worked examples
US to UK
- Given
- US size 7
- Result
- UK N, 17.35 mm
A US 7 has an inside diameter of 17.35 mm, which is a UK size N.
Measured circumference
- Given
- 54.4 mm circumference
- Result
- ≈ US 7, EU 54
The EU/ISO size is the circumference itself, so 54 mm is EU 54, which matches US 7.
Diameter to size
- Given
- 18.19 mm diameter
- Result
- US 8, UK P
Entering the diameter directly finds the closest standard size in every system.
Frequently asked questions
Wrap a thin strip of paper or string around the base of your finger, mark where it overlaps, and measure the length in millimetres. That is your inside circumference — enter it here to get your size. Alternatively, measure the inside diameter of a ring that already fits.
The ISO 8653 standard, used across much of Europe, defines ring size as the inside circumference in millimetres. So an ISO/EU size of 54 means a 54 mm circumference. It is the most direct system, with no letters or arbitrary numbers.
At the end of the day, when your fingers are at their largest and warmest. Fingers shrink in the cold and swell in heat, so a ring measured on a cold morning may feel tight later. Aim for a fit that slides on easily but resists a little coming off.
Yes. A wide band covers more of your finger and fits more snugly, so for bands wider than about 6 mm many people size up by a quarter to half a size. If you are buying a wide ring, factor that in on top of the conversion.
Fingers don’t come in standard increments, so a measured circumference often lands between sizes. The tool shows the nearest one. If you are between sizes, the usual advice is to choose the larger, especially for a knuckle that is bigger than the base of the finger.
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