About

Ring Size Tool is a free ring sizer that measures on your own screen, by first calibrating against a bank card so it knows how large a millimetre actually is on your display.

Why it exists

Because the ring size charts already online disagree with each other, and several disagree with themselves. While researching this, four well-known sizing pages gave three different UK answers for the same US size. One site’s chart page and its own downloadable PDF printed different Japanese sizes for the same measurement. Another states that a quarter US size is 0.4 mm of diameter — those are the half-size figures; a quarter size is 0.2032 mm.

None of that is malice. It is what happens when charts are copied from other charts for long enough. So nothing here is copied. Every value is computed from the published standard — ISO 8653 for EU sizes, defined from 41 to 76 mm of inner circumference; JIS S 4700 for Japan, 1 to 35; BS EN 28653 for UK letters. Each of those calculations has assertions in the repository that fail the build if a number drifts, and the site cannot deploy while they fail.

Where no standard exists — the US scale is a trade convention, and India has no standards body defining ring sizes at all — the page says so rather than presenting a number as settled.

How it works

A screen has no fixed relationship between pixels and millimetres: the same image is physically larger on a monitor than a laptop, and different again at 110% browser zoom. So the tool asks you to match an outline to a bank card, which is manufactured to ISO/IEC 7810 ID-1 at 85.60 × 53.98 mm worldwide. From that one match it knows your display’s true scale, and everything it draws afterwards is sized from it.

You then either lay a ring you own on the calibrated circle, or measure your finger with a strip of paper and type the length in. Both produce an inner diameter, which is the only measurement every national size system is actually built from.

Where it is less accurate

The arithmetic is exact; the inputs are not. A quarter US size is 0.2032 mm of diameter, so a slightly loose calibration, a ring sitting at an angle on the glass, or a paper strip pulled half a millimetre tight will all move the result. Fingers also swell and shrink through the day by roughly half a size.

So: measure more than once, and measure in the evening. If the answer lands between two sizes and the ring cannot be resized, a jeweller with a steel mandrel is more reliable than any screen, including this one. This tool is built to get you close enough to buy with confidence, not to replace a physical gauge.

Contact

Questions or corrections: get in touch. If a number here looks wrong, that is the most useful message we can receive — tell us what you measured and what the tool returned.