ISO 8653

ISO 8653 is the international standard for ring sizes. It sizes a ring by its inner circumference in millimeters, so the size is the measurement: an EU 54 ring measures 54 mm around the inside. It is the system behind EU ring sizes, and the only common one where the number needs no formula or table.

How to measure

1. Cut a narrow strip of paper, long enough to go around the finger with some overlap.
2. Wrap it around the base of the finger, flat and snug, not tight.
3. Mark where it overlaps, unroll it and measure to the mark in millimeters. That is the inner circumference, and the ISO size.
4. Check that the loop slides off over the knuckle; if it won't, measure around the knuckle too.
5. Or measure across the inside of a ring that fits, edge to edge, and multiply by 3.1416.

Fingers swell in the heat: measure at the end of the day.

On a size chart

The other systems count the same circumference their own way:

• US and Canada: circumference = 2.55 × size + 36.5 mm, in quarter sizes.
• UK, Ireland and Australia: letters, one per 1.25 mm, with C at 40 mm (BSI 1987), and half letters between.
• Japan: its own table, not a straight line.

So a 54.4 mm finger is an EU 54.5, a US 7, a UK N 1/2 and a JP 14. Ruban Size's engine takes these from ISO 8653:2016 as summarized on Wikipedia's "Ring size" page, and rounds EU sizes to the nearest half millimeter. Ring size conversion chart has the full table.

In Ruban Size

On a ring chart, Find my size takes the finger's circumference or the inner diameter of a ring that fits, explains the paper strip, and suggests the larger size between two for a wide band. The ring size converter runs the same code: