Ring Size Calculator
Turn a finger circumference or diameter in millimeters into matching US, UK, and EU ring sizes.
Wrap a strip of paper snugly around the base of your finger and measure its length in millimeters, or measure the inside diameter of a ring that already fits. Fingers swell in heat and shrink in cold — measure at the end of a warm day for the truest fit.
Sizing varies between jewelers and countries, and band width changes the fit — a wide band runs tighter, so size up a half. Treat this as a starting point and confirm with a jeweler before buying.
How it works
Ring size comes down to one number: the inside diameter of the band in millimeters. You can measure it directly from a ring that fits, or measure the circumference around your finger and let the tool divide by pi to get the diameter.
Once it knows the diameter, it looks up the closest row on the international ring-size chart. That single diameter maps to a US number, a UK letter, and an EU size (which is just the circumference in millimeters), so you get all three at once.
A quick sanity check: about 54 mm around your finger lands on roughly a US size 7. Measure warm, since fingers shrink when cold, and remember a wide band fits tighter than a thin one.
Frequently asked questions
How do I measure my finger at home?
Wrap a thin strip of paper or string snugly around the base of the finger, mark where it overlaps, and measure that length in millimeters. That length is the circumference this tool uses.
Does band width change my size?
Yes. A wide band covers more of your finger and fits more snugly, so many people size up by a half size for bands wider than about 6 mm.
Why do US, UK, and EU sizes look so different?
They're just different scales for the same physical size. The US uses numbers, the UK uses letters, and the EU number is simply the inside circumference in millimeters — this tool converts between all three.