Ring size conversion chart
Find a ring size from its inner diameter or circumference, or convert between US numbers, UK letters and EU sizes. In the EU (ISO 8653) system the size is simply the inner circumference in millimetres.
| US / Canada | UK / Australia | EU (ISO) | Inner diameter | Circumference |
|---|---|---|---|---|
| 3 | F | 44 | 14.07 mm | 44.2 mm |
| 3.5 | G | 46 | 14.47 mm | 45.5 mm |
| 4 | H | 47 | 14.88 mm | 46.8 mm |
| 4.5 | I | 48 | 15.29 mm | 48.0 mm |
| 5 | J½ | 49 | 15.69 mm | 49.3 mm |
| 5.5 | K½ | 51 | 16.10 mm | 50.6 mm |
| 6 | L½ | 52 | 16.51 mm | 51.9 mm |
| 6.5 | M½ | 53 | 16.91 mm | 53.1 mm |
| 7 | N½ | 54 | 17.32 mm | 54.4 mm |
| 7.5 | O½ | 56 | 17.73 mm | 55.7 mm |
| 8 | P½ | 57 | 18.13 mm | 57.0 mm |
| 8.5 | Q¼ | 58 | 18.54 mm | 58.2 mm |
| 9 | R½ | 60 | 18.95 mm | 59.5 mm |
| 9.5 | S½ | 61 | 19.35 mm | 60.8 mm |
| 10 | T½ | 62 | 19.76 mm | 62.1 mm |
| 10.5 | U½ | 63 | 20.16 mm | 63.3 mm |
| 11 | V½ | 65 | 20.57 mm | 64.6 mm |
| 11.5 | W½ | 66 | 20.98 mm | 65.9 mm |
| 12 | X½ | 67 | 21.38 mm | 67.2 mm |
| 12.5 | Y½ | 69 | 21.79 mm | 68.5 mm |
| 13 | Z½ | 70 | 22.20 mm | 69.7 mm |
EU (ISO 8653) sizes are the inner circumference rounded to a whole millimetre. Jewellers’ sizing can differ slightly; check with the seller.
How it’s calculated
Inner diameter (mm) = 11.63 + 0.8128 × US size · Circumference (mm) = 36.5 + 2.55 × US size · EU size = circumference in mm
Step by step: US size 7
- Diameter = 11.63 + 0.8128 × 7 = 17.32 mm.
- Circumference = 36.5 + 2.55 × 7 = 54.35 mm, about 54.4 mm (π × 17.32 = 54.41 mm).
- EU size ≈ 54; on the UK/Australian scale this is N½.
FAQ
Measure a ring that already fits on the inside edge, or wrap a strip of paper around the finger and measure the length. Fingers swell when warm, so measure at the end of the day. Jewellers’ sizing can vary slightly; check with the seller before buying.
What size is a 17 mm ring?
An inner diameter of 17 mm is about US 6.5 (16.92 mm) to US 7 (17.32 mm), UK M½ to N½, and EU 53–54.
How is the US ring size calculated?
US ring sizes grow by 0.8128 mm of inner diameter per size: diameter = 11.63 + 0.8128 × size mm. The circumference grows by about 2.55 mm per size.
What is the EU ring size?
The EU (ISO 8653) ring size is the inner circumference in millimetres. A ring with a 54.4 mm inside circumference is size 54.4, written as 54.
Do UK and Australian ring sizes match?
Yes. The UK, Ireland, Australia and New Zealand use the same alphabetical scale (A to Z, with half sizes).
Sources
- Ring size – Wikipedia (ISO 8653:2016 equivalency table): US: d = 11.63 + 0.8128 × size mm, C = 36.5 + 2.55 × size mm (this chart uses C = π × d, within 0.15 mm); UK letters from the same table.
Checked by Unitglide on 2026-10-27.