Ring Size Calculator
Enter your finger circumference or the inner diameter of a ring that fits, in millimetres or inches, and get your size in every major system — with the precise US estimate shown to one decimal.
Reviewed & updated: November 2026 — chart re-verified against ISO 8653
Find my ring size
How the calculator works
The calculator converts your measurement to an inner circumference in millimetres, estimates your US size with (circumference − 36.5) ÷ 2.55, then matches the nearest row of the standard chart for UK, EU, Japanese and Indian sizes. Diameter is circumference ÷ π. Try the on-screen sizer if you have a ring but no ruler, or the converter if you already know a size in one system.
Methodology & sources
Our calculator is built on ISO 8653:2016, which defines ring size by inner circumference in millimetres (the EU size is that circumference, rounded). US sizes are estimated with circumference = 36.5 + 2.55 × US size — so a 54.4 mm circumference gives (54.4 − 36.5) ÷ 2.55 = 7.02, a US 7 — then matched to the nearest row of the standard chart. UK letters follow BS 6820 (1.25 mm steps from C = 40 mm), which is why they are looked up rather than calculated; Japanese and Indian sizes come from published jewellers’ charts on the same millimetre base.
Charts vary by up to half a size between jewellers — millimetres are the shared truth, so confirm in mm before you buy. Full detail in how our chart is built. Reviewed and re-verified November 2026.
Ring size chart
| US / Canada | Diameter (mm) | Circumference (mm) | UK / Australia | EU (ISO) | Japan | India |
|---|---|---|---|---|---|---|
| 3 | 14.1 | 44.2 | F | 44 | 4 | 4 |
| 3.5 | 14.5 | 45.5 | G | 46 | 5 | 5 |
| 4 | 14.9 | 46.8 | H½ | 47 | 7 | 7 |
| 4.5 | 15.3 | 48.0 | I½ | 48 | 8 | 8 |
| 5 | 15.7 | 49.3 | J½ | 49 | 9 | 9 |
| 5.5 | 16.1 | 50.6 | K½ | 51 | 11 | 11 |
| 6 | 16.5 | 51.9 | L½ | 52 | 12 | 12 |
| 6.5 | 16.9 | 53.1 | M½ | 53 | 13 | 13 |
| 7 | 17.3 | 54.4 | N½ | 54 | 14 | 14 |
| 7.5 | 17.7 | 55.7 | O½ | 56 | 15 | 15 |
| 8 | 18.1 | 57.0 | P½ | 57 | 16 | 16 |
| 8.5 | 18.5 | 58.3 | Q½ | 58 | 18 | 18 |
| 9 | 18.9 | 59.5 | R½ | 60 | 19 | 19 |
| 9.5 | 19.4 | 60.9 | S½ | 61 | 20 | 20 |
| 10 | 19.8 | 62.1 | T½ | 62 | 21 | 21 |
| 10.5 | 20.2 | 63.4 | U½ | 63 | 22 | 22 |
| 11 | 20.6 | 64.6 | V½ | 65 | 23 | 23 |
| 11.5 | 21.0 | 66.0 | W½ | 66 | 25 | 25 |
| 12 | 21.4 | 67.2 | X½ | 67 | 26 | 26 |
| 12.5 | 21.8 | 68.5 | Y½ | 69 | 27 | 27 |
| 13 | 22.2 | 69.7 | Z½ | 70 | 29 | 29 |
Full chart, US sizes 3 to 13 in half sizes. EU size is the rounded circumference in mm. Japan and India share the same values on this chart.
Full detail on the ring size chart page and the ring size in mm page.
Frequently asked questions
How is US ring size calculated from circumference?
Should I measure circumference or diameter?
What if I fall between sizes?
Popular sizes in detail
One page per size — exact millimetres and inches, every equivalent, and who typically wears it: US 5 · US 6 · US 6.5 · US 7 · US 7.5 · US 8 · US 9 · US 10.