US / Canada to Inner diameter (mm) ring size
The short answer
By the formula, US / Canada 8 is Inner diameter (mm) 18.1.
Inner circumference 56.9 mm. Ring sizes are defined by that one measurement, so this conversion is exact up to rounding: US sizes are rounded to the nearest quarter, UK letters to the nearest half.
| US / Canada | Inner diameter (mm) |
|---|---|
| 3 | 14.1 |
| 3.5 | 14.5 |
| 4 | 14.9 |
| 4.5 | 15.3 |
| 5 | 15.7 |
| 5.5 | 16.1 |
| 6 | 16.5 |
| 6.5 | 16.9 |
| 7 | 17.3 |
| 7.5 | 17.7 |
| 8 | 18.1 |
| 8.5 | 18.5 |
| 9 | 18.9 |
| 9.5 | 19.3 |
| 10 | 19.7 |
| 10.5 | 20.1 |
| 11 | 20.5 |
| 11.5 | 21.0 |
| 12 | 21.4 |
| 12.5 | 21.8 |
| 13 | 22.2 |