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Ring size converter (US, UK, ISO/EU, mm)

Convert ring sizes between US/Canada, UK/Australia letters, ISO 8653 (EU) circumference and inner diameter in mm.

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Ring sizes are defined by the inside of the ring. ISO 8653, used across continental Europe, names a size by its inner circumference in millimetres, so an EU 54 ring measures 54 mm around the inside. North American sizes step by 0.032 in (0.8128 mm) of inner diameter per whole size, which gives diameter ≈ 11.63 mm + 0.8128 mm × US size; a US 7 is about 17.32 mm across and 54.4 mm around. UK and Australian letter sizes (BS 6820) step by 1.25 mm of circumference per letter, with size C at 40 mm, so N½ is about 54.4 mm.

This converter turns any of those into the others using the formulas above and rounds US sizes to the nearest quarter and UK sizes to the nearest half letter, which is how jewellers stock them. The chart lists US 3 to 13½ with the matching letter, circumference and diameter.

To find your size, measure the inside diameter of a ring that fits the same finger, or wrap a strip of paper around the finger at the knuckle and measure the length for the circumference. Wide bands fit tighter, so go up a quarter to half size for bands wider than about 6 mm.

How to use it

  1. Choose the system you know: US/Canada number, UK/Australia letter, ISO/EU circumference or inner diameter.
  2. Enter the size, for example 7, 7.25, N½ or 54.
  3. Read the equivalent sizes and check the chart for the neighbouring sizes.

Frequently asked questions

What is a US 7 ring in UK and EU sizes?

A US 7 has an inner diameter of about 17.3 mm and a circumference of 54.4 mm, which is UK N½ and EU (ISO) 54.

How do I measure my ring size at home?

Measure the inner diameter of a ring that fits in millimetres and enter it as "inner diameter", or measure your finger's circumference with a paper strip and enter it as ISO/EU. Measure in the evening, when fingers are slightly larger.

Are Australian ring sizes the same as UK sizes?

Yes. Australia, New Zealand, Ireland and the UK all use the same letter scale.

What formula does the converter use for US sizes?

Inner diameter (mm) = 11.63 + 0.8128 × US size, so US size = (diameter − 11.63) ÷ 0.8128. It matches published charts to within a tenth of a millimetre.

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