US to ISO Ring Size Chart
| US | UK | ISO / EU | JP / KR | Diameter (mm) | Circumference (mm) |
|---|---|---|---|---|---|
| 3 | F | 44 | 4 | 14.1 | 44.2 |
| 3.5 | G | 46 | 5 | 14.5 | 45.5 |
| 4 | H | 47 | 7 | 14.9 | 46.8 |
| 4.5 | I | 48 | 8 | 15.3 | 48 |
| 5 | J | 49 | 9 | 15.7 | 49.3 |
| 5.5 | K | 51 | 10 | 16.1 | 50.6 |
| 6 | L | 52 | 12 | 16.5 | 51.9 |
| 6.5 | M | 53 | 13 | 16.9 | 53.1 |
| 7 | N | 54 | 14 | 17.3 | 54.4 |
| 7.5 | O | 56 | 15 | 17.7 | 55.7 |
| 8 | P | 57 | 16 | 18.1 | 57 |
| 8.5 | Q | 58 | 18 | 18.6 | 58.3 |
| 9 | R | 60 | 19 | 18.9 | 59.5 |
| 9.5 | S | 61 | 20 | 19.4 | 60.8 |
| 10 | T | 62 | 21 | 19.8 | 62.1 |
| 10.5 | U | 63 | 23 | 20.2 | 63.4 |
| 11 | V | 65 | 24 | 20.6 | 64.6 |
| 11.5 | W | 66 | 25 | 21 | 65.9 |
| 12 | X | 67 | 26 | 21.4 | 67.2 |
| 12.5 | Y | 68 | 27 | 21.8 | 68.5 |
| 13 | Z | 70 | 28 | 22.2 | 69.7 |
| 13.5 | Z1 | 71 | 28+ | 22.6 | 71 |
| 14 | Z2 | 72 | 28+ | 23 | 72.3 |
ISO, EU, France and Germany all label rings by the inner circumference in millimetres, so the ISO / EU number is the same across Continental Europe. Japan (号) and Korea (호) share one scale. JP sizes above 28 are approximate.
From a US number to a real measurement
ISO is not an offset of the US scale, it is the ring's inner circumference in millimetres set by ISO 8653. Each step up the US column adds about 1 mm of circumference, so US 8.5 works out to ISO 58 on the table above.
ISO is the honest layer under every ring scale: ISO 8653 sets the size as the inside circumference of the band in millimetres, so the ISO column is a measurement rather than a code. The US number is a convention mapped onto it. Continental Europe labels by that same millimetre figure, which means a ring bought in France or Germany needs no conversion at all. Because the ISO value is physical, it is the safest target when a US size has to travel between countries or between a chart and a jeweller's mandrel.
Common questions
What does the ISO ring number mean?
The inside circumference of the band in millimetres, set by ISO 8653, so it is a direct measurement rather than a national code.
Is a European ring size the same as ISO?
Yes. France and Germany label by the millimetre circumference, so the ISO figure needs no further conversion in Continental Europe.
Ring size charts · How to measure ring size · Ring sizes around the world · Printable ring sizer
Sources
Ring circumference standard: ISO 8653:2016.
Japanese and Korean ring scale: rings data pack, mother-tongue sources.