How Japanese ring sizes work

Japanese ring sizes are whole numbers called 号 (gō). Size 1 has an inner diameter of 13.0 mm, and each step up adds one third of a millimeter to the diameter, which is about 1 mm of circumference. Japan 10 therefore has an inner diameter of 16.0 mm and an inner circumference of about 50.3 mm.

The international standard works differently. ISO 8653, which Japan adopted as JIS S 4700, defines a ring's size as its inner circumference in millimeters, rounded to a whole number. That number is the "EU" size you see in many shops: a ring with a 52 mm inner circumference is size 52. A few countries, such as Italy, Spain and Switzerland, subtract 40 from it. Both the Japanese number and the ISO number come from the same measurement, so you can always convert between them through the circumference.

Japan ring size chart, sizes 1 to 30

Each row shows the inner diameter and circumference for a Japanese size, and the nearest size in the other systems.

US and UK sizes are rounded to the nearest half size. For other values, use the converter above.

Some charts use a rule of thumb: the Japan number plus 40 equals the circumference in millimeters. It is close for small sizes, since Japan 1 is 40.8 mm. By size 30 it is about 1 mm too small. The chart above uses the diameter definition.

How the conversions are calculated

• Japan: inner diameter = 13 + (size − 1) ÷ 3 mm. Circumference = π × diameter.
• US: inner diameter = 0.8128 × size + 11.63 mm. A US 7 is 17.3 mm across, or 54.4 mm around.
• UK: size A has an inner circumference of 37.5 mm. Each letter adds 1.25 mm and a half size adds half of that, so N is 53.75 mm and N½ is 54.4 mm.
• EU / ISO: the inner circumference in millimeters, rounded to a whole number.

The converter turns your size into a circumference first, and then finds the nearest standard size in each system. Brands round their sizes in different ways, so a ring marked with the same size by two makers can differ a little.

How to measure your ring size at home

1. Use a ring that already fits. Measure the inside width of the ring in millimeters, from one inner edge to the opposite inner edge. Choose "Inner diameter of a ring" in the converter.
2. Use a strip of paper or string. Wrap it around the base of the finger where the ring will sit, mark the spot where it overlaps, lay it flat and measure the length in millimeters. Choose "Finger circumference".
3. Use a ring sizer. A paper, plastic or metal ring gauge, or a ring stick, shows the inner size of a ring directly.

A finger changes size a little during the day, so it is sensible to measure more than once. A wide band covers more of the finger than a thin one, and a ring has to pass over the knuckle. If your measurement falls between two sizes, try the ring on before you buy.

Buying a ring in Japan

Japanese jewelry shops and Japanese brands label rings with 号. A shop that sells a foreign brand may show the maker's own sizes instead, so check which system a chart uses before you compare numbers. If a ring does not fit, ask whether the shop can resize it. That depends on the design and the material.

For other sizes, see the Japan shoe size converter, the clothing size converter and the kids' clothing size guide. If you are shopping on a trip, the tax-free shopping guide explains the new refund system that starts on November 1, 2026.

Frequently asked questions

What does 号 mean in Japanese ring sizes?

号 (gō) is the unit for Japanese ring sizes. Size 1 has an inner diameter of 13.0 mm, and each size adds one third of a millimeter of diameter, about 1 mm of circumference.

What is a Japan size 10 in US sizes?

Japan 10 has an inner diameter of 16.0 mm and an inner circumference of 50.3 mm. That is about US 5.4, so US 5½ is the nearest size. In the UK it is close to K, and in the EU / ISO system it is 50.

Is the Japanese ring size the same as the EU size?

No. The EU / ISO size is the inner circumference in millimeters, such as 52. The Japanese number is smaller, because Japan 12 has a circumference of about 52.4 mm. Convert through the circumference, or use the chart.

What if my finger is between two sizes?

The converter shows your exact position, for example 10.4. Ring sizes come in whole steps in Japan, so try on both neighbors, or ask the shop whether a size in between is possible. Wide bands often feel tighter than thin ones.

How accurate is this converter?

It follows the published definitions for the Japanese, US, UK and ISO systems. Real rings are made by hand or machine with small differences, and brands may round their own sizes, so check the maker's chart when you can.

Sources

• JIS S 4700:1998 Ring sizes for jewellery (the Japanese version of ISO 8653:1986)
• ISO 8653:2016 Jewellery — Ring-sizes
• Wikipedia (Japanese): Ring, on the Japanese size convention
• Wikipedia: Ring size, for the US, UK and European systems

Related pages

• Japan shoe size converterJapanese cm sizes to US, UK and EU, for women and men, and back.
• Japan clothing size converterJapanese tops, dresses and pants sizes to US, UK, DE, FR and IT, with a guide to Japanese measurements.
• Japan kids' clothing size chartHeight-based Japanese sizes (80, 90, 100…) with ages, and a comparison with US sizes.
• Japan tax-free shopping (new system from November 2026)Pay tax at the register, get it back at the airport. Rules and a refund calculator.
• Japan's consumption tax: 10%, 8% and price tagsWhich rate applies, eat-in versus takeout, tax-included prices and the planned 2027 food cut.
• Japanese paper and envelope sizesA4, JIS B5, envelope numbers and the size limits for postcards and letters.

Last updated: 2026-10-04