Ring sizes: US, UK, ISO, Japan and Swiss, in mm
Unlike shoes and clothes, ring sizes are real measurements. The international standard ISO 8653 names a ring by its inner circumference in millimeters, rounded to the nearest whole number: a ring of 54.4 mm is an ISO 54. The US scale is a formula on the inner diameter, 0.458 in for size 0 plus 0.032 in (0.8128 mm) per size, so every US size converts exactly. Switzerland, Italy and Spain count the circumference minus 40 mm. Only the UK letters and Japanese numbers are fixed tables, which is why they skip or repeat around some US sizes.
| US / Canada | Inner circumference (mm) | Inner diameter (mm) | UK / Australia | Japan | Switzerland, Italy, Spain |
|---|---|---|---|---|---|
| 3 | 44.2 | 14.07 | F | 4 | 4.2 |
| 3.5 | 45.5 | 14.48 | G | — | 5.5 |
| 4 | 46.8 | 14.88 | H | 7 | 6.8 |
| 4.5 | 48 | 15.29 | I | 8 | 8 |
| 5 | 49.3 | 15.7 | J ½ | 9 | 9.3 |
| 5.5 | 50.6 | 16.1 | K ½ | 10 | 10.6 |
| 6 | 51.9 | 16.51 | L ½ | 11 | 11.9 |
| 6.5 | 53.1 | 16.92 | M ½ | 13 | 13.1 |
| 7 | 54.4 | 17.32 | N ½ | 14 | 14.4 |
| 7.5 | 55.7 | 17.73 | O ½ | 15 | 15.7 |
| 8 | 57 | 18.14 | P ½ | 16 | 17 |
| 8.5 | 58.3 | 18.54 | Q ¼ | 17 | 18.3 |
| 9 | 59.5 | 18.95 | R ½ | 18 | 19.5 |
| 9.5 | 60.8 | 19.35 | S ½ | 19 | 20.8 |
| 10 | 62.1 | 19.76 | T ½ | 20 | 22.1 |
| 10.5 | 63.4 | 20.17 | U ½ | 22 | 23.4 |
| 11 | 64.6 | 20.57 | V ½ | 23 | 24.6 |
| 11.5 | 65.9 | 20.98 | W ½ | 24 | 25.9 |
| 12 | 67.2 | 21.39 | X ½ | 25 | 27.2 |
| 12.5 | 68.5 | 21.79 | Y ½ | 26 | 28.5 |
| 13 | 69.7 | 22.2 | Z ½ | 27 | 29.7 |
How to measure
Wrap a strip of paper around the base of the finger, mark where it overlaps and measure the length in millimeters: that is the inner circumference, and its whole number is the ISO size. Measure at the end of the day when fingers are largest, and choose the larger size for a wide band or a knuckle bigger than the base. To size from a ring you already own, measure its inner diameter across the middle and find it in the diameter column.
Ring sizes are the one size system defined by measurement, so the millimeter columns are exact; the letter and Japanese columns are fixed tables. Centimeters to inches, if needed: cm to inches.
Sources
- ISO 8653:2016, Jewellery: ring-sizes, definition, measurement and designation
- Ring size comparison table (US formula, BS 6820 letters, Japanese sizes)
Diameter and circumference are computed from the definition of the US scale (inner diameter 0.458 in + 0.032 in per size, circumference = π × diameter) and checked row by row by our test suite. UK letters (British Standard BS 6820) and Japanese sizes are read from the comparison table cited above, which also confirms each computed millimeter value to within 0.05 mm. The Swiss column is the circumference minus 40 mm.
Checked against the sources on 30 October 2026. Found a difference with a current chart? The methodology page keeps the corrections log.
Other size charts
Frequently asked questions
Inner diameter 17.32 mm, inner circumference 54.4 mm, which is ISO size 54. In the UK it is an N½, in Japan a 14.
The inner circumference in millimeters, rounded to a whole number (ISO 8653). A European 52 is a ring of 52 mm around the inside, between a US 6 and 6.5.
Inner diameter in mm = 11.633 + 0.8128 × US size. Multiply the diameter by π for the circumference. US 9: 18.95 mm diameter, 59.5 mm circumference.