Ring size conversion: US to UK, EU (ISO) and more
Short answer: a US 7 is about an N in the UK and Australia[1 · Oct 29, 2026] and about 54 on the ISO (European) scale, which is simply the inner circumference in millimetres.[2 · Oct 29, 2026]
How each system works
- US and Canada: numbers with quarter and half sizes; each whole size is 0.032 in (0.8128 mm) of diameter.[2 · Oct 29, 2026]
- UK, Ireland, Australia: letters with half sizes. Since BS 6820:1987, one letter step equals 1.25 mm of circumference.[2 · Oct 29, 2026]
- ISO 8653 (most of continental Europe): the size is the inner circumference in mm.[2 · Oct 29, 2026]
- Italy, Spain, Switzerland: circumference in mm minus 40.[2 · Oct 29, 2026] Some retailer charts print slightly different Swiss values, so confirm with the seller.
- Germany and the Netherlands: often the inner diameter in mm.[2 · Oct 29, 2026]
Conversion chart
| US / Canada | UK / Australia (GIA chart) | ISO / EU (circumference, rounded) | Italy / Spain (C − 40) | Diameter in mm |
|---|---|---|---|---|
| 3 | F | 44 | 4.2 | 14.1 |
| 3.5 | G | 45 | 5.5 | 14.5 |
| 4 | H | 47 | 6.8 | 14.9 |
| 4.5 | I | 48 | 8.0 | 15.3 |
| 5 | J | 49 | 9.3 | 15.7 |
| 5.5 | K | 51 | 10.6 | 16.1 |
| 6 | L | 52 | 11.9 | 16.5 |
| 6.5 | M | 53 | 13.1 | 16.9 |
| 7 | N | 54 | 14.4 | 17.3 |
| 7.5 | O | 56 | 15.7 | 17.7 |
| 8 | P | 57 | 17.0 | 18.1 |
| 8.5 | Q | 58 | 18.2 | 18.5 |
| 9 | R | 60 | 19.5 | 18.9 |
| 9.5 | S | 61 | 20.8 | 19.4 |
| 10 | T | 62 | 22.1 | 19.8 |
| 10.5 | U | 63 | 23.3 | 20.2 |
| 11 | V | 65 | 24.6 | 20.6 |
| 11.5 | W | 66 | 25.9 | 21.0 |
| 12 | X | 67 | 27.2 | 21.4 |
| 12.5 | Y | 68 | 28.5 | 21.8 |
| 13 | Z½ | 70 | 29.7 | 22.2 |
The math (our own arithmetic): d = 11.63 + 0.8128 × US size; C = π × d; ISO = C rounded to a whole mm; Italy/Spain = C − 40.[2 · Oct 29, 2026] Example for US 7: d = 17.32 mm, C = 54.4 mm, ISO ≈ 54, Italy/Spain ≈ 14.4. UK letters are taken from GIA's international chart, not calculated.[1 · Oct 29, 2026]
Charts from different jewelers can differ by a fraction of a size because of rounding. When ordering from another country, give the seller your measurement in millimetres as well as the size.
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Sources: 1. GIA 4Cs, How to determine ring size. 2. Wikipedia, Ring size. Checked Oct 29, 2026.