Methodology

Four systems are defined by a rule that can be written as arithmetic: the US scale, the UK letters, ISO 8653 and the Swiss, Italian and Spanish numbers. The pages for each name the source, the rule and the day it was checked. The US rule is checked against the 43-row chart that GIA publishes, and the largest difference is a few tenths of a millimetre, which is rounding.

Japan, Hong Kong, China and India have no published formula we could confirm. Where a page shows them, they are tables copied from the charts named on the page, and the page says they are a trade convention that charts can disagree on by a size. A size a source does not print is shown as not published.

Every size is turned into the inside circumference in millimetres, which is the unit of ISO 8653, and the other systems are searched for the nearest size. The inside diameter is the circumference divided by 3.1416. Each system has its own steps, so the nearest size is usually a little larger or smaller than the one you started from.

The difference in millimetres is shown instead of hidden. A size is called exact only when the gap is under 0.15 mm. In the charts a size marked with ≈ is the closest one in that system, and the caption says the largest gap in the chart.

A web page cannot read the physical size of a screen, so the sizer asks you to match an outline to the short edge of a bank card, 53.98 mm (ISO/IEC 7810 ID-1). From that it keeps the number of CSS pixels in a millimetre and draws the circle at the ring’s inside diameter. The calibration is stored only in your browser and is dropped when the zoom, the pixel ratio or the screen changes.

The accuracy given on the page, about 0.3 mm of inside diameter, is an estimate from the geometry: placing each end of the outline within about one pixel of the card edge and judging the ring’s inside edge within about 0.2 mm. It has not been measured on a large set of devices, which is why the page offers a coin check and points to the printable sizer.

The PDFs are drawn in vector in millimetres, so a print at 100 percent puts every mark where it says. A box the size of a bank card and a 100 mm ruler let you check the print before you use it. The paper strip allows for 0.1 mm office paper: the scale is printed on the outside of the loop and the finger touches the inside, so the marks are moved by 2 × 3.1416 × 0.1 mm. Thicker card reads a little large.

The date on a page is the day its facts or its words last changed, the same date the sitemap reports. Rebuilding the site without changing a page leaves its date alone.

Reports of errors go through the contact page. A confirmed correction changes the data, the page and its date together. Brand pages copy what each brand prints and the day it was read; a brand may have changed its chart since.