Ring size chart: US, EU (ISO) and Japan in mm

How to use the chart

1. If you have a ring that fits, measure its inner diameter in millimetres, straight across the middle.
2. Choose inner diameter in the finder and enter the value. The finder highlights the first size that is large enough.
3. If you know a size in one system, choose that system and enter the size.
4. For an expensive ring, have your finger measured by a jeweller.

The first chart has one row for each US half size, the second one row for each European size. Without JavaScript the finder is unavailable, but both charts are complete.

How the sizes are derived

• European size, ISO 8653: the inner circumference in millimetres. Size 54 has a circumference of 54 mm and a diameter of 54 divided by 3.1416, which is 17.19 mm.
• Japanese size: size 1 has an inner diameter of 13 mm, and each size adds one third of a millimetre.
• US and Canadian size: no official definition exists. The chart uses a straight line through the gauge dimensions that the US National Bureau of Standards reported in 1921: about 0.488 inch for size 1 and 0.877 inch for size 13. Each size adds about 0.82 mm of diameter.

The US column was compared with the chart of the Gemological Institute of America and agrees within 0.1 mm of diameter for every half size from 3 to 13.5.

British letter sizes are not in the chart. The British standard that defined ring sizes is withdrawn, we could not read its text, and the published charts we compared disagree with each other.

Worked example

A ring that fits has an inner diameter of 18.0 mm.

1. Circumference: 18.0 times 3.1416 is 56.5 mm. The European size is 56 or 57.
2. Japan: 18.0 minus 13 is 5; times 3 is 15; plus 1 is size 16.
3. US: the finder selects the first row that is large enough, US 8 with 18.2 mm. US 7.5 has 17.7 mm and would be tight.

How accurate is a conversion?

The systems use different steps: about 0.82 mm of diameter per US size, one third of a millimetre per Japanese size and about 0.32 mm per European size. A size in one system therefore falls between two sizes of another, and charts round differently. Expect differences of one European or Japanese size between charts.

A chart gives a starting point. Trying on, or a measurement by a jeweller, decides.

Common mistakes

• Measuring the outside of the ring.
• Measuring a ring that is bent or not round.
• Reading a strip of paper or string wrapped around the finger as exact. One millimetre of circumference is more than a third of a US size.
• Confusing diameter and circumference. European sizes are circumferences.
• Treating a converted size as exact when ordering a ring that cannot be resized.

Sources and checks

The values in the chart are calculated from the definitions in the sources below. They are not copied from another chart. Last checked against the sources on September 28, 2026.

ISO sizes are the inner circumference in millimetres, so diameter is size divided by pi. The calculated diameters match the five values in the preview of ISO 8653:2016.

Japanese sizes are calculated from the rule of 13 mm for size 1 and one third of a millimetre per size.

US sizes have no official definition. The US column is a straight line through the gauge dimensions reported by the National Bureau of Standards in 1921. It agrees with the GIA chart within 0.1 mm of diameter for every half size from 3 to 13.5.

UK letter sizes are left out: the defining standard is withdrawn, its text was not available, and published charts disagree.

Conversions between systems are rounded to the nearest size and can differ by one size from the chart of a jeweller. Have the finger measured by a jeweller before an expensive purchase.

• ISO 8653:2016, Jewellery: ring-sizes, definition, measurement and designation (official preview at iTeh Standards)

Used for: The ring size is the inner circumference in millimetres; sizes 41 to 76; diameters of the measuring stick for sizes 41, 51, 61, 71 and 76.

• CEN and CENELEC, EN ISO 8653:2026 ring size

Used for: The ISO ring size standard was adopted as a European standard in 2026.

• Platinum Guild International Japan, ring size column

Used for: Japanese size 1 has an inner diameter of 13.0 mm and each size adds about one third of a millimetre; sizes 10 and 20 are 16.0 mm and 19 1/3 mm.

• National Bureau of Standards, Circular 43, second edition, 1921 (NIST digital archive)

Used for: No official dimensions of US ring sizes were found; gauge makers reported 0.485 and 0.491 inch for size 1 and 0.877 inch for size 13.

• GIA, How to determine ring size (ring size chart)

Used for: Inside diameters in millimetres for US sizes 3 to 13.5, used to check the calculated US column.