How to measure a finger without a sizer
Cut a strip of paper about a centimetre wide, wrap it round the base of the finger, mark where it overlaps and measure the marked length with a ruler. That length is the circumference in millimetres, and it is the number everything else comes from. Do not use thread: it stretches, and half a millimetre is a quarter of a size.
If there is already a ring that fits, the other route is easier and more reliable: measure its inside diameter across the widest point. Measuring the outside includes the thickness of the band twice over, which is a size and a half of error.
Each country counts something different
- Spain, France and much of Europe use the inside circumference in millimetres minus forty. A 54 mm finger is a size 14.
- Italy, Switzerland and the Netherlands use the circumference minus thirty-nine, so their numbers sit one above the Spanish ones.
- The United States and Canada count the inside diameter: size 0 is 11.63 mm and each size adds 0.8128 mm, which is exactly a thirty-second of an inch.
- The United Kingdom, Ireland and Australia use letters, from A upwards in half steps, on the circumference. It is the only alphabetical scale in use.
Because they measure different things, converting one scale into another without going back to the millimetres chains two roundings. Every scale here is worked out from the measurement of the finger, which is the one thing they have in common.
The formulas, to do it by hand
- Circumference measured: 54 mm.
- Spanish and French: 54 − 40 = 14.
- Diameter: 54 ÷ π = 17.19 mm.
- American: (17.19 − 11.63) ÷ 0.8128 = 6.84, so a 6¾.
What is worth knowing
Fingers change size more than people expect. Heat swells them and cold shrinks them, salt and exercise swell them, and the difference between morning and evening can reach half a size. The sensible measurement is taken in the afternoon at a normal temperature, and if you fall between two sizes, go up: a ring has to pass the knuckle, which is wider than the base of the finger.
Band width matters too. A wide band sits against more skin and feels tighter than a thin one of the same size, so for anything above six or seven millimetres, jewellers usually go up a quarter or half a size. And a ring can be resized up or down a couple of sizes if it is a plain band, but not if it has stones set all the way round.