Ring Size Converter
Find your ring size from a US size, a measured diameter, or a UK letter size, plus the matching circumference in millimeters.
| US size | UK letter | Diameter (mm) | Circumference (mm) |
|---|---|---|---|
| 7 | N | 17.3 | 54.3 |
Half-size letter mappings vary a little between charts. If you have a ring that already fits, measuring its inside diameter is the most reliable way to size a new one.
How it works
There are two ways to use this. Either start from a US ring size you already know, or start from a measured inside diameter (in millimeters) and let the tool find the closest matching size. Both paths pull from the same published chart, which pairs each US size with its inside diameter, a UK letter size, and the circumference around that diameter.
Worked example: a US size 7 ring has an inside diameter of 17.3mm, which is a UK N. Multiply that diameter by pi and you get a circumference of about 54.3mm, which is the length of string or paper you'd need to wrap once around the inside of the band. If you instead measured an existing ring at 18.0mm across, the closest chart row is 18.1mm, which is a US 8 and a UK P.
FAQ
How do I measure a ring I already own?
Lay the ring flat and measure straight across the inside edge, from one inner wall to the opposite inner wall, in millimeters. That's the inside diameter. Enter it in the diameter mode above and the tool will match it to the nearest standard size.
Why do UK sizes use letters instead of numbers?
UK, Australian, and several other regions size rings by letter (A through Z) tied to circumference, while the US and Canada use a numeric scale tied to diameter. Both systems describe the same physical ring, just with a different label.
Is a half size worth chasing down?
Often yes for comfort, especially with wider bands, which tend to fit snugger than a thin band in the same labeled size. If you're between two sizes on the chart, most jewelers can size a ring up or down by a half size after the fact.
Do these letter mappings match every jeweler's chart?
Close, but not guaranteed exact. Different charts round the half sizes slightly differently, so if you're ordering something that can't be resized later, confirm the exact diameter with the seller rather than relying on the letter alone.
For more on choosing and wearing jewelry with an outfit, see how to accessorize any outfit, quiet luxury and the understated look, and the old money aesthetic explained.