The scales
- US, Canada, Mexico: inner diameter in mm = 11.63 + 0.8128 × size (size 0 = 0.458 in; one size = 0.032 in). Rings are stocked in quarter sizes. US 7 = 17.32 mm across, 54.4 mm around.
- UK, Ireland, Australia, New Zealand: inner circumference = 37.5 + 1.25 × letter index (A = 0 … Z = 25), half sizes + 0.625 mm, from BS 6820:1987 (size C = 40 mm). UK L = 51.25 mm around.
- EU / ISO 8653 / France: the inner circumference in mm itself (ISO 8653:2016). Stocked in whole and half millimetres.
- Japan and East Asia (trade scale): inner diameter = 13 + (size − 1) ÷ 3 mm; size 14 = 17.33 mm. Secondary sources disagree on whether JIS S 4700 numbers rings by diameter or circumference; the charts jewellers use follow this rule, and so do we.
Always: circumference = π × diameter; 1 inch = 25.4 mm.
Nearest size and the gap
A measurement becomes a circumference c. US: exact size = (c ÷ π − 11.63) ÷ 0.8128, rounded to the nearest quarter. UK: (c − 37.5) ÷ 1.25, rounded to the nearest half. EU: c rounded to the nearest half millimetre. Japan: (c ÷ π − 13) × 3 + 1, rounded to the nearest whole. The gap shown in brackets is c minus the nearest size’s own circumference; under 0.3 mm we print “exact”. Example: US 7 (54.4 mm) → UK N½ (+0 mm), EU 54.5, Japan 14.
Measuring with a strip
The length of a strip wrapped snugly round the finger is the circumference. With a larger knuckle, the tool uses the circumference halfway between the base and the knuckle, a common jewellers’ rule for a ring that must pass the knuckle and not spin.
The on-screen sizer
Screens differ in pixels per millimetre, so the sizer asks you to match a dashed box to a bank card, which every issuer makes to 85.60 × 53.98 mm (ISO/IEC 7810 ID-1). The box’s width in pixels divided by 85.60 gives pixels per millimetre, and the circle is then drawn at the chosen diameter times that figure. Browser zoom changes the figure, so it must stay at 100% after calibrating.
The printable sizer
The sheet is an SVG whose units are millimetres, with the page size set to A4, so a print at 100% is at true size. Each circle has the exact inner diameter of its US size; the strip is marked at each size’s circumference; the same bank-card box checks the print.
The chart
The chart runs US 3 to 15 in half sizes, each row at the US size’s exact diameter and circumference, with the nearest UK, EU and Japanese sizes beside it. Some UK half sizes are skipped between rows, because the scales’ steps differ (US half sizes 1.28 mm, UK half sizes 0.625 mm, EU 1 mm, Japan 1.05 mm of circumference); above Z the UK scale continues as Z+1 to Z+6.
What is a trade figure, not a standard
The average sizes (women US 6 to 7, men US 9 to 10, usual ranges 5 to 8 and 8 to 12) are what jewellers report selling, not a measured population survey. The width rule (a quarter size up at 6 mm, a half at 8 mm), the knuckle rule, and the size changes through the day are jewellers’ practice. The Japanese scale’s relation to JIS S 4700 is disputed, as noted above.
What this leaves out
Comfort-fit bands feel about a quarter size looser than flat ones; very wide bands need more than the rule above; rings in tungsten, titanium and ceramic cannot be resized; and a jeweller’s steel gauge on the actual finger beats any measurement here. Jewellers’ printed charts differ from one another by a few tenths of a millimetre because each rounds the scales its own way.