Quick answer
Inside diameter is the straight line across the inside of a ring. Circumference is the distance around the inside. Both can be converted into standard ring sizes.
MM conversion chart
| US | Diameter | Circumference | Approx. UK | Approx. EU |
|---|---|---|---|---|
| 5 | 15.7 mm | 49.3 mm | J 1/2 | 49 |
| 6 | 16.5 mm | 51.9 mm | L 1/2 | 52 |
| 7 | 17.3 mm | 54.4 mm | N 1/2 | 54 |
| 8 | 18.1 mm | 57.0 mm | P 1/2 | 57 |
| 9 | 19.0 mm | 59.5 mm | R 1/2 | 60 |
| 10 | 19.8 mm | 62.1 mm | T 1/2 | 62 |
How to measure diameter
Place a ring on a ruler and measure only the empty inside space, not the metal. For best results use calipers or the diameter to size converter.
How to measure circumference
Wrap non-stretch string around the finger, mark the overlap, then measure the string in millimeters. Enter the value into the circumference converter.
Common mistakes
Do not measure the outer diameter of a ring, do not stretch string, and do not measure cold fingers. Small errors in millimeters can move you by half a size or more.
Frequently asked questions
Is diameter or circumference better?
Diameter is easiest when measuring an existing ring. Circumference is easier when measuring a finger with string or paper.
What does EU ring size mean in mm?
EU sizes roughly represent inner circumference in millimeters, so EU 54 is close to 54 mm around the inside of the ring.
Can I convert mm to US ring size?
Yes. Use the chart below or the MM to ring size converter for a nearest-size recommendation.
