Ring Size Converter
Convert a ring size between the US numeric scale, UK letter scale, European ISO size, inside diameter and inside circumference. Ring sizing is unusually confusing because the US uses half and quarter numbers, the UK uses letters, and much of Europe simply labels rings by their inside circumference in millimeters. This tool ties them together using the standard relationship that inside diameter in millimeters equals 11.63 plus 0.8128 times the US size, with circumference being diameter times pi. Measure an existing ring across the inside, or wrap a strip of paper around your finger to get the circumference, then enter it here to find your size everywhere. Sizing near a half step is common, so if you land between two values choose the larger for a comfortable fit. All math runs locally in your browser, and the equivalents update the instant you type.
How to use Ring Size Converter
- Choose the system you know such as US, UK, EU, diameter, or circumference.
- Enter your value.
- Read every equivalent ring size and measurement.
- Use the diameter to check against an existing ring.
Frequently asked questions
Which ring systems does it cover?
It converts across US, UK, EU, and the physical diameter and circumference of the band, so any one value reveals all the others.
How are diameter and circumference related?
Circumference equals diameter times pi, so a ring with a 17 mm inside diameter has about a 53.4 mm circumference. The tool handles this conversion automatically.
Why do US sizes use half steps?
US ring sizes advance in half sizes, each roughly 0.4 mm of diameter apart, which is why entering a precise measurement can land between whole sizes.
What is Ring Size Converter?
Convert a ring size between the US numeric scale, UK letter scale, European ISO size, inside diameter and inside circumference. Ring sizing is unusually confusing because the US uses half and quarter numbers, the UK uses letters, and much of Europe simply labels rings by their inside circumference in millimeters.
Is Ring Size Converter free to use?
Yes. Ring Size Converter is completely free, with no sign-up and no usage limits.
Does Ring Size Converter work in a web browser?
Yes. Ring Size Converter runs in any modern web browser. There is nothing to download or install.
Is my data private with Ring Size Converter?
Yes. Ring Size Converter runs entirely on your device in your browser, so nothing you enter is uploaded to a server.
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