Units
Ring size in millimetres
Millimetres are the unit every ring size system is built from. But “my ring is 17 mm” is ambiguous until you say which millimetre number you mean — diameter or circumference — because they differ by a factor of π.
Two different mm numbers — do not mix them up
Inner diameter (mm): the straight line
The distance straight across the inside of the ring, through its centre. For wearable rings it runs from about 14 mm to about 23 mm. If your number is in this range, it is almost certainly a diameter. A US 7 ring has an inner diameter of 17.32 mm.
Inner circumference (mm): the way around
The distance around the inside of the ring — the length a finger measurement actually gives. For wearable rings it runs from about 44 mm to about 71 mm, and it is exactly π times the diameter. A US 7 ring has an inner circumference of 54.4 mm. The EU size is this number, rounded to a whole millimetre.
The quick test: a number under 25 mm is a diameter; a number between 40 and 75 mm is a circumference. If someone tells you “size 54”, that is a circumference (EU 54, US 7) — not a 54 mm diameter, which would not fit any finger.
The full table, in millimetres
| US / Canada | Inner diameter (mm) | Inner circumference (mm) | EU / ISO |
|---|---|---|---|
| 3 | 14.07 | 44.2 | 44 |
| 3.5 | 14.48 | 45.5 | 46 |
| 4 | 14.88 | 46.7 | 47 |
| 4.5 | 15.29 | 48.0 | 48 |
| 5 | 15.70 | 49.3 | 49 |
| 5.5 | 16.10 | 50.6 | 51 |
| 6 | 16.51 | 51.9 | 52 |
| 6.5 | 16.92 | 53.2 | 53 |
| 7 | 17.32 | 54.4 | 54 |
| 7.5 | 17.73 | 55.7 | 56 |
| 8 | 18.14 | 57.0 | 57 |
| 8.5 | 18.54 | 58.2 | 58 |
| 9 | 18.95 | 59.5 | 60 |
| 9.5 | 19.35 | 60.8 | 61 |
| 10 | 19.76 | 62.1 | 62 |
| 10.5 | 20.17 | 63.4 | 63 |
| 11 | 20.57 | 64.6 | 65 |
| 11.5 | 20.98 | 65.9 | 66 |
| 12 | 21.39 | 67.2 | 67 |
| 12.5 | 21.79 | 68.5 | 69 |
| 13 | 22.20 | 69.7 | 70 |
| 13.5 | 22.61 | 71.0 | 71 |
Diameter steps are about 0.41 mm per half-size; circumference steps are about 1.3 mm per half-size. That is why circumference is the easier number to measure by hand: the same half-size is three times as large in circumference as in diameter, so a 1 mm reading error matters less.
Worked examples
- Ruler reads 17.32 mm across a ring. That is a diameter: US 7, EU 54, circumference 54.4 mm.
- Tape reads 51.9 mm around a finger. That is a circumference: US 6, EU 52, diameter 16.51 mm.
- A seller lists “size 62”. That is an EU size, i.e. a 62 mm circumference: US 10, diameter 19.76 mm.
Related guides
- Ring diameter calculator
Convert a diameter in mm, cm or inches to every system.
- Ring circumference calculator
Convert a circumference in mm, cm or inches to diameter and sizes.
- Ring size in centimetres
When your reading is in cm, not mm — and why rounding matters.
- Ring size in inches
When your reading is in inches, and why fractions mislead.
- International ring size chart
The same table with UK, Australia and India added.