Ring size converter

The formulas

US and Canada: size 0 is 11.63 mm across the inside, and each whole size adds 0.8128 mm (0.032 inch). EU sizes follow ISO 8653: the size is the inside circumference in millimetres, which is the diameter times π. UK, Ireland and Australia use letters from A at 37.5 mm circumference, each letter 1.25 mm more, with half sizes between (British Standard BS 6820).

A worked example

US 7 is 11.63 + 7 × 0.8128 = 17.32 mm inside. Its circumference is 17.32 × π = 54.41 mm, which rounds to EU 54.5. From A at 37.5 mm that is (54.41 − 37.5) ÷ 0.625 ≈ 27 half steps, which is UK N½.

In the studio

When you open a ring in Jart, Design Facts finds the largest circle that fits inside the band, from the mesh itself, and reports its diameter with the US and EU sizes from these same formulas. If the number is not what you expected, check the file's units and that the band is closed all the way round.

Measuring a finger

Measure at the end of the day, when fingers are largest, and not when cold. The ring has to pass the knuckle, so measure there as well and take the larger size. Wide bands sit tighter than narrow ones; many jewellers go up a quarter or half size for a band over 6 mm. A ring sizer from your jeweller is more reliable than a strip of paper.