Measurement methodology

Formulae

• Inner circumference = π × inner diameter.
• Inner diameter = inner circumference ÷ π.
• EU / ISO size = inner circumference in millimetres, rounded to the nearest whole millimetre.
• Screen calibration: pixels per millimetre = card-rectangle width in pixels ÷ 85.6 mm.
• Screen diameter: circle diameter in pixels ÷ pixels per millimetre.

Calibration reference

Standard payment cards follow ISO/IEC 7810 ID-1: 85.60 mm × 53.98 mm. That is why a bank card, not a coin or a ruler held at an angle, is the calibration object. Place it flat against the screen, not tilted. The rectangle's height follows the card's aspect ratio automatically, so matching the width also checks the height. No other reference object is supported: other objects are only added if their dimensions are independently verified and recorded here.

CSS pixels are never assumed to equal millimetres. The only bridge is the calibration above. Browser zoom, a display change, or device rotation changes the pixel-to- millimetre ratio, so the online ring sizer records the display (viewport width, visual scale, device pixel ratio) at calibration and warns — hiding the previous result — if any of them changes, until the user recalibrates.

Central sizing dataset (v2)

One version-controlled dataset drives every tool: data/ring-sizes.ts (the unified US-anchored chart: US, UK approx., Australia approx., EU / ISO, India approx., diameter, circumference) and data/size-systems.ts (the native ladders each country page shows: UK whole letters, EU 44–72, India 8–24, Australia J–U). The chart, the converter and the calculators all resolve a measurement to the same unified row, so they cannot disagree for the same measurement. The full source list, with every unresolved difference, is in docs/SIZE_SOURCES.md: UK disagrees by a half-letter across publishers, Australia by up to about 0.3 mm for the same letter, India by ±1 size, and Japan, Switzerland, Brazil, China and Russia are deliberately not shipped as definitive columns because no consistently corroborated ladder was found for them.

Central measurement engine

Geometry lives in lib/geometry.ts (card dimensions, pixels per millimetre, circle pixels to diameter, fine-step conversion, display-change detection) and is reused by every tool. Conversion lookup lives separately in data/ring-sizes.ts and lib/sizing.ts, which consume millimetre values from the engine. No tool has its own copy of the geometry, so tools cannot drift apart.

Chart and tolerance

The US half-size ladder is anchored at US 3 = 14.07 mm inner diameter, rising about 0.41 mm per half size. Because a half size is so small, even a small matching error can shift the result by half a size — so the tool reports the nearest table row and flags between-size readings instead of inventing extra precision. No accuracy percentage is claimed here: no controlled accuracy study has been run yet (see Accuracy and limitations), and a typical-error figure would be invented without one.

What is approximate

• UK letters: simplified and approximate. Published UK mappings disagree by a half-letter in places.
• Australia: Australian charts commonly use UK letters, but that mapping has not been independently verified for this site, so the online ring sizer marks it as requiring validation rather than presenting it as verified.
• Japan: not shown as a column. Its published tables disagree by 1–2 sizes at larger sizes, so it is omitted rather than invented. India IS shown, but only as approximate (circumference − 40, ±1 size) — see the India country page and the chart's India column.
• Band-width sizing-up advice: common jeweller guidance, not a standard.
• Not in this stage: camera measurement, brand charts, individual-size pages. (A printable PDF sizer does exist — see Printable Ring Sizer.) No accuracy claim beyond this page is made anywhere on the site: results are estimated, not exact, certified, or laboratory-level.
• No advertising, no camera access, and no account system. Analytics exists but is inactive unless configured and consented to (see Privacy policy).