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About RingSizePro
Free sizing tools for anyone buying jewelry online, for themselves or as a gift, in any country.
How the numbers are calculated
Every ring conversion starts from one physical quantity, the ring's inside circumference, and is calculated from the published scale definitions rather than copied from someone else's chart:
- ISO 8653:2016 defines ring sizes as inside circumference in millimetres (continental Europe).
- US / Canada: size 0 = 0.458 in (11.63 mm) inside diameter; each size adds 0.032 in (0.8128 mm).
- BS 6820:1987 (UK, Ireland, Australia): size C = 40 mm circumference, 1.25 mm per letter.
- Italy, Spain, Switzerland: circumference minus 40 mm.
- Japan / China and India: non-linear scales taken from the widely used equivalency table (as compiled in Wikipedia's "Ring size" article) and cross-checked against a Japanese jeweler's published chart.
- Bangles: the Indian 2 + n/16 inch convention, cross-checked against a major Indian jeweler's published sizes.
- Bank-card calibration: ISO/IEC 7810 ID-1 cards measure 85.60 × 53.98 mm.
Bracelet allowances and necklace positions follow common jeweler practice and are labelled as guidance, not standards. Brands vary; when a retailer publishes its own chart, follow it for that purchase.
Why "one size" isn't universal
Different countries settled on different rulers: an inch-based diameter scale in North America, a letter scale in Britain, plain millimetres in Europe. The converter lets you step between them.
The sizing desk
The Ask page uses an AI model (via Vercel AI Gateway) instructed with the same formulas. It is there for situations a chart can't cover, and it can make mistakes.