How Japanese ring sizes work
Japanese ring sizes are whole numbers written with the character 号 (gō), for example 10号. Size 1 is a ring with an inner diameter of 13.00 mm (about 40.8 mm circumference). Each size up adds one third of a millimetre of diameter, or about 1.05 mm of circumference.
Because each step is a third of a millimetre, every third size lands on a whole millimetre: size 1 is 13 mm, size 4 is 14 mm, size 7 is 15 mm, size 10 is 16 mm, size 13 is 17 mm, size 16 is 18 mm, size 19 is 19 mm, size 22 is 20 mm and size 25 is 21 mm.
The formula
- Diameter (mm) = 13 + (size − 1) ÷ 3
- Size = 3 × (diameter − 13) + 1, rounded to the nearest whole number
Japanese ring size chart
| Japanese size (号) | Inner diameter (mm) | Inner circumference (mm) | Nearest US size |
|---|---|---|---|
| 1 | 13.00 | 40.8 | — |
| 2 | 13.33 | 41.9 | — |
| 3 | 13.67 | 42.9 | — |
| 4 | 14.00 | 44.0 | 3 |
| 5 | 14.33 | 45.0 | 3½ |
| 6 | 14.67 | 46.1 | 4 |
| 7 | 15.00 | 47.1 | 4 |
| 8 | 15.33 | 48.2 | 4½ |
| 9 | 15.67 | 49.2 | 5 |
| 10 | 16.00 | 50.3 | 5½ |
| 11 | 16.33 | 51.3 | 6 |
| 12 | 16.67 | 52.4 | 6 |
| 13 | 17.00 | 53.4 | 6½ |
| 14 | 17.33 | 54.5 | 7 |
| 15 | 17.67 | 55.5 | 7½ |
| 16 | 18.00 | 56.5 | 8 |
| 17 | 18.33 | 57.6 | 8 |
| 18 | 18.67 | 58.6 | 8½ |
| 19 | 19.00 | 59.7 | 9 |
| 20 | 19.33 | 60.7 | 9½ |
| 21 | 19.67 | 61.8 | 10 |
| 22 | 20.00 | 62.8 | 10 |
| 23 | 20.33 | 63.9 | 10½ |
| 24 | 20.67 | 64.9 | 11 |
| 25 | 21.00 | 66.0 | 11½ |
| 26 | 21.33 | 67.0 | 12 |
| 27 | 21.67 | 68.1 | 12 |
| 28 | 22.00 | 69.1 | 12½ |
| 29 | 22.33 | 70.2 | 13 |
| 30 | 22.67 | 71.2 | — |
A dash means no US half size falls within 0.2 mm of that Japanese size. A Japanese step (0.33 mm) is finer than a US half size (about 0.4 mm), so some Japanese sizes sit between two US sizes. Some international charts list Japanese sizes one number lower above US 8½; this chart follows the Japanese diameter standard.
Converting between Japanese and US sizes
Common examples: 7号 is about US 4, 9号 is about US 5, 13号 is about US 6½, 16号 is about US 8 and 19号 is about US 9. When a Japanese size falls between two US sizes, choose the larger one for wide bands and the smaller one for thin bands.
Buying rings from Japanese shops
- Japanese online shops list sizes in 号. Give them your size in 号, or your inner diameter in millimetres, rather than a US size.
- Some Japanese brands only make whole sizes, so a US quarter size may not exist. Use the diameter column above to pick the closest size.
- Check whether the shop offers resizing before you buy. Rings with stones set all the way round, such as eternity bands, usually cannot be resized.
How to find your Japanese size at home
Measure the inner diameter of a ring that fits well, then use the formula or the chart above. You can also use the Ring Sizer app to measure the ring on your phone screen and read the size in millimetres.
Frequently asked questions
What does 号 mean on a Japanese ring?
号 (gō) is the counter used for ring sizes in Japan. 10号 simply means size 10.
What is Japanese size 10 in US sizes?
Size 10 is a 16.0 mm inner diameter, which is about US 5½.
What is Japanese size 1 in millimetres?
Size 1 is 13.00 mm inner diameter, about 40.8 mm circumference. It is a very small size, used mainly for children's and pinky rings.
Are Japanese and Korean ring sizes the same?
They are often very close, but charts differ between shops. Compare your inner diameter in millimetres rather than relying on the number alone.
