Ring Size Converter
Convert between US, UK, EU ring sizes, and inner diameter.
Last updated
How to convert ring sizes
- Choose the system you know: US, UK or the inner diameter in millimetres.
- Pick your size.
- Read the matching US and UK sizes, the diameter, and the circumference used for EU sizes.
About this tool
Diameter is the real underlying measurement — circumference is always exactly diameter × π, computed here rather than looked up, so those two numbers are always mathematically exact relative to each other. US and UK size labels come from a standard published reference chart mapping diameter to a size label; the "EU" figure shown here is the ring's circumference in millimetres, which is how the French/Swiss sizing convention actually defines a ring size (other regional systems exist and can differ by a few points, so confirm against the specific system your ring's origin uses if it matters).
Nothing you select here is sent to a server — the entire tool runs as JavaScript in your browser tab.
Questions about ring sizes
How do I find my ring size at home?
Measure the inside diameter of a ring that fits, in millimetres, and enter it. Or wrap a strip of paper round your finger, measure its length and divide by 3.14 for the diameter.
What is the difference between US and UK ring sizes?
US sizes are numbers (such as 7) and UK sizes are letters (such as O). Both describe the inside size of the ring.
How do EU ring sizes work?
An EU size is the inside circumference in millimetres, so a ring with a 54 mm circumference is EU size 54.
When is the best time to measure?
At the end of the day when your fingers are warm, as fingers are smaller in the cold. Make sure the ring can still pass over your knuckle.