A ring size that is correct in one country can be unrecognisable in another. A US 7 is a UK N½, an ISO 54 and a Japanese 14. This converter takes a size in any one system and gives the others, together with the inside circumference and diameter in millimetres, which is the measurement every system is built on.
How to use the ring size converter
- Choose the system your size is in: US, UK letters, ISO/EU, Japan, Italy/Spain/Switzerland or German diameter.
- Type the size. Use a number such as 7 or 7.5, or a UK letter such as N or N 1/2.
- Read the results. The other systems are shown, with the inner circumference and diameter.
- For a purchase, check the shop’s own chart.
Conversion formulas
circumference = π × diameter = π × (13 + (JP − 1) ÷ 3) = π × German size
All the systems are defined as functions of the inside circumference, so the converter first finds the circumference and then writes it in the other systems. The rules come from the Wikipedia ring size article, which states the ISO 8653 relationship to the US scale, the British standard of 1987 and the Swiss and German conventions, and from the Japanese tables of Moocota and Luft (retrieved 2026-10-15).
| US size | Inner circumference (mm) | Inner diameter (mm) | ISO / EU | UK | Japan | Italy / Spain / Switzerland |
|---|---|---|---|---|---|---|
| 3 | 44.1 | 14.1 | 44 | F 1/2 | 4 | 4 |
| 4 | 46.7 | 14.9 | 47 | H 1/2 | 7 | 7 |
| 5 | 49.2 | 15.7 | 49 | J 1/2 | 9 | 9 |
| 6 | 51.8 | 16.5 | 52 | L 1/2 | 11 | 12 |
| 7 | 54.3 | 17.3 | 54 | N 1/2 | 14 | 14 |
| 8 | 56.9 | 18.1 | 57 | P 1/2 | 16 | 17 |
| 9 | 59.5 | 18.9 | 59 | R 1/2 | 19 | 19 |
| 10 | 62.0 | 19.7 | 62 | T 1/2 | 21 | 22 |
| 11 | 64.5 | 20.5 | 65 | V 1/2 | 24 | 25 |
| 12 | 67.1 | 21.4 | 67 | X 1/2 | 26 | 27 |
| 13 | 69.7 | 22.2 | 70 | Z 1/2 | 29 | 30 |
Worked example
A US 7 ring, ordered from a European shop.
- Circumference: 2.55 × 7 + 36.5 = 54.35 mm, so ISO 54.
- Diameter: 54.35 ÷ π = 17.3 mm.
- Italy / Spain / Switzerland: 54 − 40 = 14.
- UK: N 1/2; Japan: 14.
Cautions when converting
- Half sizes: ISO sizes are whole millimetres, so a US 7 and a US 7¼ can land on the same ISO size.
- UK letters: some jewellers use different letter-to-millimetre tables, so check before ordering.
- Width: a wide band needs a larger size than a thin band of the same number.
- Not every system is linear: Wikipedia says Indian, Japanese and Chinese scales have no simple linear link to circumference. The Japanese JCS scale used here follows the Moocota and Luft tables (steps of a third of a millimetre of diameter); other East Asian charts may differ.
To find a size from your finger, use the Ring Size Calculator. Other accessories: bracelet-size-calculator and Hat Size Calculator. This converter is general information and does not replace a ring gauge.
Sources
Every figure above comes from these pages, read directly on the dates shown. Brand names are used only to identify published charts.
- Wikipedia — Ring size (ISO 8653, US, UK, Swiss and German systems) — checked 2026-10-15
- Tiffany & Co. — ring size guide — checked 2026-10-15
- Moocota — Japanese ring size chart — checked 2026-10-15
- Luft — ring size converter (JCS) — checked 2026-10-15
Frequently asked questions
How do I convert a US ring size to EU?
The EU (ISO) size is the inside circumference in millimetres. Multiply the US size by 2.55 and add 36.5: a US 7 is 54.35 mm around, so about ISO 54. The converter does this and the reverse.
How do I convert UK ring letters?
UK sizes step by 1.25 mm of circumference per letter, with C at 40 mm. N is 53.75 mm and N 1/2 is 54.375 mm. Some old charts are based on diameter and differ by up to about half a millimetre.
How is a Japanese ring size calculated?
Japan size 1 has an inside diameter of 13 mm and each size adds one third of a millimetre of diameter, about 1 mm of circumference. Size 9 is therefore 15.67 mm across and 49.2 mm around, or roughly US 5.
What do the Swiss and Italian sizes mean?
Wikipedia explains that in Italy, Spain and Switzerland the size is the circumference minus 40 mm, so ISO 50 is size 10. The German and Dutch system uses the diameter in millimetres instead.
Why does my result differ from a shop’s chart?
Shops round to whole or half sizes and use their own tables. For example the same UK letter can be 0.2 to 0.6 mm different between charts, so for a purchase prefer the shop’s chart. See also Ring Size Calculator.