Inner diameter (mm) to UK / AU ring size
The short answer
By the formula, Inner diameter (mm) 18 is UK / AU P.
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) | UK / AU |
|---|---|
| 14 | F |
| 14.5 | G 1/2 |
| 15 | H 1/2 |
| 15.5 | J |
| 16 | K |
| 16.5 | L 1/2 |
| 17 | M 1/2 |
| 17.5 | O |
| 18 | P |
| 18.5 | Q 1/2 |
| 19 | S |
| 19.5 | T |
| 20 | U 1/2 |
| 20.5 | V 1/2 |
| 21 | X |
| 21.5 | Y |
| 22 | Z 1/2 |