Ring Size International Converter
Pick your sizing system and enter a ring size to convert it worldwide.
Result
How it is calculated
You choose the system your size is in (US/Canada, UK/Australia, EU/ISO in mm, Japan, or a plain inner diameter in mm) and type the size. The tool first converts it to the inner circumference in millimetres, then derives everything else from that. For US sizes circumference = 36.537 + 2.5535 × size; for EU/ISO the size is the circumference itself; Japan is 40 + size; a diameter is multiplied by π. Diameter is circumference ÷ π. The output lists circumference, diameter and the US, EU and Japan sizes; UK is an input only. UK sizes are letters, so the tool needs a number: enter the position of the letter in the alphabet (A = 1, N = 14), using a fitted linear formula, so treat UK results as close estimates. Any circumference below 36 mm or above 82 mm, roughly US 0 to US 17, gives no result. Japanese sizes start at 1 (41 mm), so for smaller rings the Japan line shows a dash.
Worked example
You wear a US size 7. Circumference = 36.537 + 2.5535 × 7 = 54.41 mm, shown as 54.41 mm. Inner diameter = 54.41 ÷ π = 17.32 mm. The EU/ISO size is the circumference rounded to one decimal, 54.4, and the US size reads back 7.0. Japan size is the circumference minus 40, 14.41, which the tool shows as 14. Check it against the retailer's own conversion table before ordering.
Common mistakes
- Choosing the wrong system in the dropdown. Entering 7 as an EU size means a 7 mm circumference, which is far outside real ring sizes, so the tool shows no result rather than a size.
- Typing a UK letter such as N. Only the alphabet position (14) is accepted, and a plain letter returns no result.
- Confusing inner diameter with outer diameter or with finger circumference. The diameter option is the inside of the ring in millimetres; measure a ring that fits, not the outside of the band.
- Measuring the finger in the cold or in the morning, when it is thinner. Ring size is usually taken at the end of the day, in warm conditions.
- Treating the converted Japan, UK or half sizes as exact. Because they come from fitted formulas, round to the nearest size on the retailer's chart; for wide bands, many jewellers advise going up a half size.
About this calculator
Ring sizes aren't standardized globally — the US/Canada system uses a numeric scale (roughly 3 to 13, often with half sizes), the UK/Australia system uses letters (A through Z+), the EU/ISO system uses the finger's inner circumference directly in millimeters, and Japan uses its own numbered scale — and all four ultimately trace back to the same underlying physical measurement: the ring's inner circumference (or equivalently, inner diameter), since that's what actually determines fit regardless of which country's chart you're reading.
This calculator lets you pick your known sizing system and a size, then converts it via that shared circumference/diameter measurement to the inner circumference and diameter plus the equivalent US, EU/ISO and Japan sizes. For UK sizes, enter the letter's position as a number (A = 1, B = 2 and so on); letters can't be typed in, and UK letter sizes aren't given as a result. Online jewelry shoppers ordering from an overseas retailer, jewelers fulfilling international orders, and people buying a ring as a gift while traveling abroad use it to avoid ordering the wrong size when a size "7" in the US doesn't mean anything in a UK or Japanese size chart.
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