About
Online Ring Size is a free tool for measuring your ring size on screen and converting it into every country’s sizing standard. It exists because almost every other on-screen sizer stops at a bare number, and a bare number is the one thing a ring size shouldn’t be.
A ring that is half a size out still doesn’t fit. It spins, or it won’t clear a knuckle, and either way it goes back. The gap between a good sizer and a bad one is not the circle drawn on the screen — that part is trivial. It’s everything around it: whether the screen is calibrated at all, whether the calibration can be checked, whether band width and knuckles are accounted for, and whether the tool is honest about the error left over at the end.
We started by measuring the competition rather than the fingers. Most tools drew a circle at an arbitrary scale and hoped. Several asked for a credit card and offered no alternative. One shipped a conversion chart with a size listed twice. None of them printed a tolerance. So this was built the other way round: start from the published standards, make calibration verifiable, and put the error bar on the screen next to the answer.
Each of the 11 national standards is implemented as the formula its own standards body published — ISO 8653 for the European circumference scale, JIS S 4700 for Japan, NBR 16058 for Brazil, the 0.8128 mm-per-size US trade scale, the Wheatsheaf letter scale for Britain. Nothing is a hand-keyed lookup table. Typed tables drift and pick up errors; we found a duplicated size in a competitor's chart while researching ours.
Every result carries a stated accuracy range, and we tell you which step is limiting it. Calibration error is proportional and measurement error is absolute, so we combine them in quadrature — the standard way of adding independent uncertainties. That range is the most useful thing on the results screen, and most sizers don't show it at all.
Most on-screen sizers assume a credit card. Card ownership is not universal, and a tool that assumes it isn't really global. You can calibrate against a coin from your own currency, a folded sheet of A4 or Letter, a ruler, or your device's published pixel density — then verify the scale with a second object.
The whole calculation runs in your browser. There is no account and no upload — no measurement, calibration value or result is ever sent to us. Your calibration is saved in your own device's local storage so you don't have to redo it, and clearing your browser data removes it. We count page views with Google Analytics, which sees that a page was opened and nothing about what you measured on it.
At a typical screen density, one pixel of misalignment is already about a fifth of a US size. That is a physical floor no amount of arithmetic gets past. For a ring you cannot return, we say plainly: get sized on a steel mandrel at a jeweller. Use this to know what to expect before you walk in.
The business model
It’s a fair question, and the honest answer is that the site is cheap to run. It’s a set of static files with no database, no servers doing work per visit, and nothing loading on the page besides a page-view counter. Hosting a few hundred pre-built HTML pages costs very little, and the calculation happens on your device rather than ours.
There is no advertising, no affiliate link to a jeweller, and nothing collected about you to sell. If that ever changes, it will be stated on this page and in the privacy policy before it happens — not quietly, after.