Inner diameter (mm) to Swiss / IT / ES ring size
The short answer
By the formula, Inner diameter (mm) 18 is Swiss / IT / ES 17.
Inner circumference 56.5 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.
| Inner diameter (mm) | Swiss / IT / ES |
|---|---|
| 14 | 4 |
| 14.5 | 6 |
| 15 | 7 |
| 15.5 | 9 |
| 16 | 10 |
| 16.5 | 12 |
| 17 | 13 |
| 17.5 | 15 |
| 18 | 17 |
| 18.5 | 18 |
| 19 | 20 |
| 19.5 | 21 |
| 20 | 23 |
| 20.5 | 24 |
| 21 | 26 |
| 21.5 | 28 |
| 22 | 29 |