Ring Size Conversion: US, UK, EU and Japanese Sizes Compared
Rings from international brands and overseas shops are labeled "US 6", "UK L" or "EU 52", and Japanese brands use yet another number, the Japanese size (gou). This guide puts US, UK, EU (ISO) and Japanese ring sizes side by side in a conversion chart, explains how each system is defined, why conversions drift apart, and how to avoid mistakes when buying from another country.
Each country counts ring sizes differently
Ring sizes are expressed differently by country and region. They all describe the inside of the ring, but they differ in whether they count the inner circumference or the inner diameter, where they start from and how many millimeters each step adds.
| System | Main regions | How it's defined | One size step |
|---|---|---|---|
| Japanese size (gou) | Japan | Inner circumference (mm) − 40 | 1 mm of circumference |
| US | United States, Canada | A number set from the inner diameter in inches (in quarter sizes) | About 2.55 mm of circumference |
| UK | UK, Australia and others | Letters (A–Z, with half sizes) | 1.25 mm of circumference |
| EU (ISO) | Most of Europe | The inner circumference in mm as the number | 1 mm of circumference |
Japanese and EU sizes both go up in 1 mm steps of circumference, so converting between them is easy: subtract 40 from the EU size to get the Japanese size (EU 52 → Japanese 12). A US size step, on the other hand, is about 2.5 Japanese sizes wide, and UK steps are finer than Japanese ones, so conversions don't always line up exactly.
Conversion chart: Japanese, US, UK and EU sizes
Starting from Japanese sizes, here are the US, UK and EU sizes calculated with the standard formulas. US sizes are in quarter steps and UK sizes include half sizes; each shows the nearest value.
| Japan | Inner circumference | US | UK | EU |
|---|---|---|---|---|
| 5 | 45 mm | 3¼ | G | 45 |
| 7 | 47 mm | 4 | H½ | 47 |
| 9 | 49 mm | 5 | J | 49 |
| 10 | 50 mm | 5¼ | K | 50 |
| 11 | 51 mm | 5¾ | K½ | 51 |
| 12 | 52 mm | 6 | L½ | 52 |
| 13 | 53 mm | 6½ | M | 53 |
| 14 | 54 mm | 6¾ | N | 54 |
| 15 | 55 mm | 7¼ | O | 55 |
| 17 | 57 mm | 8 | P½ | 57 |
| 19 | 59 mm | 8¾ | R | 59 |
| 21 | 61 mm | 9½ | S½ | 61 |
| 23 | 63 mm | 10¼ | U | 63 |
| 25 | 65 mm | 11¼ | W | 65 |
Going the other way, from US to Japanese: US 4 ≈ 7, US 5 ≈ 9, US 6 ≈ 12, US 7 ≈ 14, US 8 ≈ 17, US 9 ≈ 20 and US 10 ≈ 22. Each whole US size equals about 2.5 Japanese sizes, so US 6 falls right between Japanese 11 and 12.
How US sizes are defined: from the inner diameter in inches
US sizes come from the inner diameter in inches. The commonly used formula is "inner diameter (in) = 0.458 + 0.032 × size", or in millimeters "inner diameter (mm) ≈ 11.63 + 0.8128 × size". Each size up adds 0.8128 mm (just over 1/32 inch) to the inner diameter, or about 2.55 mm to the circumference.
That step equals about 2.5 Japanese sizes, which is too coarse to fit well with whole sizes alone, so quarter and half sizes (6¼, 6½ and so on) are normal in the US. For converting inches and centimeters in general, see converting inches and centimeters.
How UK and EU sizes are defined
- UK sizes: A is an inner circumference of 37.8 mm, and each letter adds 1.25 mm. Half sizes such as "B½" sit between letters, so you can choose in 0.625 mm steps. The same system is used in Australia, New Zealand and elsewhere.
- EU sizes (ISO 8653): the inner circumference in millimeters, used directly as the number. EU 52 is a 52 mm inner circumference, the same as Japanese size 12. You'll often see it on French and German brands.
Why conversions differ by a quarter to half a size
Compare conversion charts online and you'll find the same US 6 listed as Japanese 11 in one and 12 in another. There are several reasons.
- Different step sizes: one US size is about 2.5 Japanese sizes, so rounding one way or the other shifts the result by one
- Different formulas: charts differ in whether they work from the diameter or the circumference, and how precisely they round
- How each brand makes rings: even with the same label, the actual inner diameter can vary by 0.1–0.2 mm between brands and methods
Treat conversion charts as a guide. When buying from another country, comparing the inner circumference or inner diameter in millimeters is the most reliable method. In the Ring Size Measurer, "Convert a size" takes a Japanese size, US size, EU size or inner diameter and shows all the other sizes at once.
Tips for buying from international brands and overseas shops
- 1
Know your inner circumference in millimeters
Note down your inner circumference (or diameter) in millimeters as well as your size, and you can convert to any country's system. For how to measure, see how to measure your ring size without a ring sizer.
- 2
Check the brand's own size chart
Many brands publish a size chart with circumference and diameter on their official site. Use the brand's chart over a general conversion chart.
- 3
When in doubt, go up a size
If a conversion falls between two sizes, choose the larger one. For a wide band, consider going up another half size.
- 4
Check the resizing and exchange terms
Returns and exchanges from overseas shops cost shipping and take time. Some designs and materials can't be resized, so check before you buy.
FAQ
- What Japanese size is a US 7 ring?
- US 7 has an inner circumference of about 54.4 mm, roughly Japanese size 14. Some charts and brands list it as 14–15.
- What US size is Japanese size 9?
- Japanese size 9 (49 mm inner circumference) is about US 5. In UK sizes it's J, and in EU sizes 49.
- What Japanese size is a UK L?
- UK L has an inner circumference of about 51.6 mm, between Japanese 11 and 12. UK L½ is about Japanese 12.
- How do I convert an EU size to a Japanese size?
- EU (ISO) sizes are the inner circumference in mm, so subtract 40 to get the Japanese size. EU 52 is Japanese 12, and EU 57 is Japanese 17. Some brands use their own systems, though, so confirm the circumference on their size chart.
- Why do conversion charts give different sizes?
- One US size equals about 2.5 Japanese sizes, so rounding can shift the result by one; charts also use different base formulas, and actual inner diameters vary by brand. Comparing the inner circumference or diameter in millimeters is the most reliable way.
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