Inside diameter and circumference in millimetres for every ring size — the universal measurement system understood by all jewellers.
Ring dimensions are most accurately expressed in millimetres — specifically the inside diameter and inside circumference of the ring. These measurements are universally understood regardless of which country's sizing system is in use. Providing dimensions in mm is the most reliable way to communicate ring size internationally.
The inside diameter is the measurement across the widest point of the inside of the ring — from one inner edge to the other, measured in a straight line through the centre. This is the most commonly used dimension when measuring an existing ring.
The inside circumference is the total measurement around the inside of the ring — equivalent to what you measure when you wrap paper or string around your finger. Circumference = diameter × π (3.14159).
| Diameter (mm) | Circumference (mm) | US / Canada | UK / IRE / AUS | EU | Japan |
|---|---|---|---|---|---|
| 14.1 | 44.2 | 3 | F | 44 | 4 |
| 14.5 | 45.6 | 3.5 | G | 45 | 5 |
| 14.9 | 46.8 | 4 | H | 46.5 | 7 |
| 15.3 | 48.0 | 4.5 | I | 48 | 8 |
| 15.7 | 49.3 | 5 | J | 49 | 9 |
| 16.1 | 50.6 | 5.5 | K | 50.5 | 11 |
| 16.5 | 51.9 | 6 | L | 51.5 | 12 |
| 16.9 | 53.2 | 6.5 | M | 53 | 13 |
| 17.3 | 54.4 | 7 | N | 54 | 14 |
| 17.7 | 55.7 | 7.5 | O | 55.5 | 16 |
| 18.2 | 57.2 | 8 | P | 57 | 17 |
| 18.5 | 58.2 | 8.5 | Q | 58 | 18 |
| 18.9 | 59.3 | 9 | R | 59 | 19 |
| 19.4 | 61.0 | 9.5 | S | 61 | 21 |
| 19.8 | 62.2 | 10 | T | 62 | 22 |
| 20.2 | 63.5 | 10.5 | U | 63.5 | 24 |
| 20.7 | 65.0 | 11 | V | 65 | 25 |
| 21.1 | 66.2 | 11.5 | W | 66 | 26 |
| 21.5 | 67.5 | 12 | X | 67.5 | 28 |
| 21.9 | 68.8 | 12.5 | Y | 68.5 | 29 |
| 22.3 | 70.1 | 13 | Z | 70 | 30 |
| 22.7 | 71.3 | 14 | Z+2 | 71 | 31 |
Set the ring on a flat, well-lit surface with the opening facing up.
Use a mm ruler or digital calliper. Measure from one inner edge to the directly opposite inner edge. This is the diameter.
Look up your measurement in the Diameter column above to find the ring size in any system.
Cut a thin strip of paper and wrap it around the base of your ring finger.
Mark where it overlaps. Measure the length in mm. This is your circumference.
Use the Circumference column above to find your ring size.
An important nuance: when a ring has a comfort-fit profile (rounded interior), it behaves as if slightly larger than its nominal dimensions suggest. A comfort-fit ring with a given inner diameter will feel looser than a flat-interior ring of the same diameter. Additionally, a wider band contacts more of the finger and creates more friction, making the ring feel tighter.