Ring Size Converter
Enter a US ring size and we give you the inner diameter, the circumference and the EU size.
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The formula
Inner diameter (mm) = 11.63 + 0.8128 x US size. Circumference = π x diameter.
How to find your ring size
- Wrap a thin strip of paper or string around the base of your finger.
- Mark where it overlaps and measure the length in millimetres. That is your circumference.
- Enter the size in this tool to compare, or divide the circumference by 3.14 for the diameter.
Common sizes
| US size | Diameter | Circumference |
|---|---|---|
| 5 | 15.7 mm | 49.3 mm |
| 6 | 16.5 mm | 51.9 mm |
| 7 | 17.3 mm | 54.4 mm |
| 8 | 18.1 mm | 57.0 mm |
| 9 | 18.9 mm | 59.5 mm |
Tips
Measure at the end of the day, when fingers are at their largest, and avoid measuring when your hands are cold. Wider bands fit more tightly, so go up half a size for a band over 6 mm. If you are between sizes, choose the larger.
Examples to check
Type these into the calculator above to see how it behaves. Every result below was produced by the same calculator code that runs on this page.
| What you enter | What you get |
|---|---|
| US ring size: 5 | Inner diameter: 15.69 mm |
| US ring size: 7 | Inner diameter: 17.32 mm |
| US ring size: 9 | Inner diameter: 18.95 mm |
Ring Size Converter: frequently asked questions
Are ring sizes the same everywhere?
No. The US, UK, EU and Japan each use different scales. The diameter and circumference in millimetres are the reliable way to compare.
How exact is the conversion?
It uses the internationally agreed definitions, and rounds only the number you see.
Is what I enter in this converter saved or sent anywhere?
No. The calculation runs in your browser, so what you enter is not sent to our servers. We only count, anonymously, whether a tool ran successfully.
Is this converter free to use?
Yes. It is free, needs no sign-up and has no usage limit.
Last reviewed: October 3, 2026