Ring Size Converter
Find your ring size in US, EU (ISO) and Swiss/Italian systems from a finger measurement or an existing ring.
About the Ring Size Converter
ISO 8653, used across most of Europe, numbers rings by inside circumference in millimetres. US and Canadian sizes rise by 2.55 mm of circumference (0.8128 mm of diameter) per whole size, starting from 36.5 mm at size 0. Swiss, Italian and Spanish sizes are the circumference minus 40.
UK, Japanese and Indian sizes use their own tables that aren't a simple formula, so check a jeweller's chart for those.
The formula
US size = (circumference mm − 36.5) ÷ 2.55
- EU size
- circumference in mm
- Diameter
- circumference ÷ π
Common ring sizes
| US | Diameter (mm) | Circumference / EU |
|---|---|---|
| 4 | 14.87 | 46.7 |
| 5 | 15.68 | 49.3 |
| 6 | 16.49 | 51.8 |
| 7 | 17.3 | 54.3 |
| 8 | 18.11 | 56.9 |
| 9 | 18.92 | 59.5 |
| 10 | 19.74 | 62 |
| 11 | 20.55 | 64.6 |
| 12 | 21.36 | 67.1 |
How to read your result
- Measure at the end of the day, when fingers are largest.
- For wide bands, go up a quarter to half a size.
Worked example
Example: US / Canada size = 8.75
- Inputs: I know: Finger circumference (mm); Value: 59.
- Formula: US size = (circumference mm − 36.5) ÷ 2.55
- Result: US / Canada size = 8.75
Questions people ask
What's the most common ring size?
Around US 6–7 for women and 9–10 for men.
Can I measure with string?
Yes, but string stretches; a paper strip is more accurate. Measuring an existing ring's inside diameter is best.