Ring Size Converter
Measure the inside of a ring that fits and read off the matching US and EU sizes.
Measure what fits
The most reliable input is the inner diameter of a ring you already wear on the right finger — measured in millimetres.
Sizes are approximate
Scales differ slightly between jewellers and countries. Treat the result as a close starting point, not a guarantee.
What is a ring size converter?
From millimetres to US and EU sizes
A ring size converter turns one physical measurement — the inner diameter of the band in millimetres — into the size numbers different countries use. The US uses a numeric scale, while the EU/ISO scale is simply the inner circumference of the ring in millimetres. Enter the diameter you measured and the converter gives you both, so you can shop confidently across regions.
First turn the diameter into a circumference, then read off each scale. The circumference is the diameter times π; the EU/ISO size is that circumference in millimetres; the US size is a linear function of it.
circumference = diameter × π · EU size = circumference (mm) · US size = (circumference − 36.537) ÷ 2.551The EU/ISO scale needs no extra arithmetic — it is the circumference itself. Only the US size uses the offset-and-slope formula, which jewellers derive from standard mandrel measurements.
Suppose a ring that fits has an inner diameter of 17.35 mm.
Find the circumference
17.35 × π = 54.5 mm — this is also the EU/ISO size.Subtract the offset
54.5 − 36.537 = 17.97.Divide by the slope
17.97 ÷ 2.551 = a US size of about 7.
The EU/ISO size is the easiest to trust: it is the inner circumference in millimetres, so EU 54.5 means a band 54.5 mm around the inside. The US number is the conventional American size for that circumference. To get an accurate input, measure an existing ring's inside diameter rather than wrapping a finger — a worn ring already accounts for knuckle clearance. Note that the UK and several Commonwealth countries use letters (J, K, L…) rather than numbers, so a UK size will not match the US figure directly.
The arithmetic is exact, but real-world fit has tolerances to keep in mind.
Why the converted size is approximate
Sizing scales vary slightly between jewellers and countries, so a converted size can land half a size off a particular shop's chart. Measure when your fingers are at a normal temperature — they shrink when cold and swell when warm — and remember that a wide band fits more snugly than a thin one at the same nominal size. For an expensive or non-resizable ring, confirm with a jeweller before ordering.