PickGlobeRing size chart and converter
Pick a size in one system and see it in all the others, with the inside diameter in millimetres.
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- The ISO size is the inside circumference in millimetres; US and UK sizes are older trade scales.
- UK letters differ between charts by up to a full letter, so treat the result as within half a size.
- Measure a ring that fits you, or print the sizer below at 100% scale.
How we calculate
We turn any ring size into the inside circumference in millimetres, then convert that to each system. The ISO size is the circumference itself; the US size comes from the diameter; the UK letter from the circumference.
- US and Canada: inside diameter = 11.633 mm + 0.8128 mm per size. A size 7 is 11.633 + 7 x 0.8128 = 17.32 mm across, so 54.4 mm around.
- ISO and EU: the size is the circumference. A size 7 US is about 54.4 mm, so ISO 54.4 (a ring stick would show 54). France uses the same number; Switzerland, Italy and Spain subtract 40 (14.4); Germany and the Netherlands use the diameter (17.3).
- UK, Ireland, Australia, NZ: the circumference is 37.5 mm + 1.25 mm per letter from A. N is 53.75 mm, and 54.4 mm falls on N 1/2.
- Japan: diameter = 13.0 mm + (size - 1) / 3 mm. A diameter of 17.3 mm is about size 14.
Three worked examples: a US 6 is 16.51 mm across and 51.9 mm around, about UK L 1/2 and ISO 51.9. A UK P is 56.25 mm around, 17.9 mm across, between US 7.5 and 8. A finger strip that reads 60 mm is a US 9 (60 / pi = 19.1 mm, about 9.2).
UK charts do not agree: some put US 7 at N, others at N 1/2, one retailer puts N at 53.2 mm. We use the BS 6820:1987 scale as reported by secondary sources, so a UK result can be half a letter off the seller's own chart. We could not read the ISO or BS texts themselves (the ISO one is paid), so the ISO definition is taken from the catalogue summaries and the CEN-CENELEC notice. Check the seller's size guide and returns before you buy.
How the systems work
| Item | Rule |
|---|---|
| ISO / EU (ISO 8653) | The size is the inside circumference in millimetres. ISO ring sticks run from 41 to 76. France uses the same number; Switzerland, Italy and Spain use the circumference minus 40; Germany and the Netherlands use the inside diameter in mm. |
| US and Canada | A trade convention, not a published standard: size 0 is 11.63 mm across and each whole size adds 0.8128 mm of diameter (0.032 in). Half sizes are common. |
| UK, Ireland, Australia, NZ | Letters from A, with half letters. We use the BS 6820:1987 scale as reported by secondary sources: 1.25 mm of circumference per letter, N = 53.75 mm. Charts in use differ by up to a full letter, so treat the letter as within about half a size. |
| Japan | Numbers (go): diameter = 13.0 mm + (size - 1) / 3 mm. This is a trade convention; we found no published standard. |
| Above Z | Z+1 and Z+2 are the same 1.25 mm step carried on, not read in any standard. Treat them as approximate. |
The UK now has BS EN ISO 8653:2026, which uses the ISO circumference. The A to Z letter scale continues as trade practice.
Checked on 27 October 2026.
| Item | Rule |
|---|---|
| Best method | A metal ring gauge from a jeweller is the most reliable. String and paper can stretch or tear and miss the knuckle. |
| Knuckle | The ring has to pass over the knuckle, which is often wider than the base of the finger. Check both. |
| Time and temperature | Fingers are smaller in the cold and larger in the heat, and change through the day. Measure when your hand is warm and repeat on another day. |
| Which hand | Measure the finger that will wear the ring. The dominant hand is often about half a size larger. |
| Wide bands | Wider bands usually need a slightly larger size. For wedding bands wider than about 4 to 5 mm, ask the jeweller. |
| Our printable sizer | Print at 100% (no fit to page). Check that the card outline matches a real bank card (85.6 x 54 mm) and the ruler line measures 5 cm. |
Checked on 27 October 2026.
Sources and last checked
- How each ring size system works, checked 27 October 2026: ISO 8653:2016 Jewellery: ring sizes, definition, measurement and designation; CEN-CENELEC: EN ISO 8653:2026 adopted as a European standard (May 2026); BSI: BS 6820:1987 ring sizes (withdrawn 1993; letter scale kept as trade practice); Wikipedia: Ring size (formulas for US, UK and European systems; secondary source); GIA 4Cs: How to determine ring size (chart and measuring advice, 18 Jan 2018); Adin Antique Jewellery: global ring size guide (cross-check of UK letters); Luft: Japanese ring size converter (JCS formula, trade convention)
- Measuring your finger, checked 27 October 2026: GIA 4Cs: How to determine ring size (chart and measuring advice, 18 Jan 2018); GIA Quality Assurance Benchmark: accurate determination of finger and ring size; ISO/IEC 7810:2019 ID-1 card, 85.60 x 53.98 mm (used for the print-scale check)
Ring sizes are approximate. Charts and jewellers differ by up to a full letter in the UK system, fingers change with temperature and time of day, and wide bands fit tighter. Check the seller's own size guide and returns before you buy. Also: Printable ring sizer. Read more: Buying jewelry online: metals and hallmarks.