Inner diameter (mm) to EU / ISO ring size
The short answer
By the formula, Inner diameter (mm) 18 is EU / ISO 57.
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) | EU / ISO |
|---|---|
| 14 | 44 |
| 14.5 | 46 |
| 15 | 47 |
| 15.5 | 49 |
| 16 | 50 |
| 16.5 | 52 |
| 17 | 53 |
| 17.5 | 55 |
| 18 | 57 |
| 18.5 | 58 |
| 19 | 60 |
| 19.5 | 61 |
| 20 | 63 |
| 20.5 | 64 |
| 21 | 66 |
| 21.5 | 68 |
| 22 | 69 |