Ring Size Converter
Convert ring sizes between US numbers, inside diameter, circumference and UK letters — from a measurement or a known size.
How it works
Inside diameter (mm) = US size × 0.8128 + 11.63 · Circumference = diameter × π · UK letter position ≈ US size × 2
US ring sizes are a linear scale tied to the ring’s inside diameter: each full size adds about 0.8128 mm (a 32nd of an inch) of diameter, starting from roughly 11.63 mm at size 0. From the diameter, the circumference — the distance around the inside, and what a soft measuring tape reads — is just π times the diameter. UK sizes use letters, which advance about two letters per US size, so they map to an alphabet position. All ring-sizing systems carry small inconsistencies between makers, so this is a close guide rather than a guarantee.
Worked example
A US size 7 has an inside diameter of 7 × 0.8128 + 11.63 ≈ 17.3 mm and a circumference of 17.3 × π ≈ 54.4 mm. Its UK position is about 7 × 2 = 14, or roughly the letter N.
Frequently asked questions
How do I measure my ring size at home?
Wrap a strip of paper or string snugly around the base of your finger, mark where it overlaps, and measure the length in millimetres — that is the inside circumference. Enter it here to get the US size and diameter. Measure at the end of the day when fingers are largest, and warm.
Why do UK sizes use letters?
The UK and much of the Commonwealth use an alphabetical scale (A, B, C …) where each letter is a small step in circumference, roughly two letters per US whole size. Because it is a different scale, the letter mapping here is approximate — confirm with a jeweller’s sizer for an important purchase.
Should I size up for wide bands?
Yes — wider bands fit more snugly, so many jewellers suggest going up a quarter to a half size for bands over about 6 mm. Cold weather and time of day also change finger size, so aim for a fit that is comfortable but firm enough not to slip off.
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This calculator is for educational purposes. Double-check important results before acting on them.