About

About Real Sizer

Real Sizer is a ruler that runs in your browser and can be made true. You hold a bank card against the glass, drag an outline until the edges agree, and from that moment a centimetre on the screen is a centimetre of glass, to within a stated band. Before you do that it is not: CSS fixes an inch at exactly 96 pixels whatever your panel is, and no browser has ever been told how large a display actually is. Every on-screen ruler starts from the same 96-pixel assumption. This one says so.

Three rules, and they are load-bearing

A number means exactly what the words beside it say. A mark labelled 10 mm is a claim, and a claim needs a band. So the readout names its calibration state and the band it earned, the band is computed from the object you actually matched rather than from a house constant, and a graduation the display cannot resolve is dropped and named instead of being drawn anyway. Where an option is worse — the short edge of a card, a screen too coarse for half-millimetre marks — it is worse by a number, not by a hedge.

Nothing leaves your device. There is no upload, no file picker, no camera, no clipboard read and no account anywhere on this site. The whole thing is static files and arithmetic in your tab. One record is kept in your own browser's storage so a calibration survives to your next visit, and what that record contains is listed field by field. The site's own security policy blocks the page from talking to any outside origin at all, so this is a property your browser enforces rather than a sentence we wrote.

The limits are published, at length. The methodology page spends more words on what this tool cannot do than on what it can, because that is the honest proportion for a measuring instrument. It names the failure no browser can detect — a swap to a different monitor of the same resolution, which is a third wrong with every signal identical — and it says which reference objects were verified and withheld anyway, and why.

What is deliberately missing

There is no database of device screen sizes here, and no automatic detection of your display's density. Both exist elsewhere, and both produce a confidently wrong number under exactly the conditions where nobody can notice: an iPhone with Display Zoom on reports a resolution belonging to a different phone, a Mac on a scaled resolution reports a framebuffer rather than a panel, and privacy modes rewrite those values deliberately. A guess wearing the badge of a measurement is the one thing this site is built to avoid, so the screen size you can type in is used only to move the outline near where it belongs before you drag it.

There is also only one reference object offered as a primary — the ID-1 bank card — with a sheet of A4 as a second opinion. Fourteen more are carried in the site's data with their nominal sizes, and refused by name until their tolerances have been read out of a published source. Two of them were verified and are withheld anyway. The calibration page lists all sixteen with the reasoning.

Who runs it

The site is built and run by an independent developer, and it will be paid for by advertising in the written parts of the pages. None is being shown yet: the places advertising will go are marked in the markup and are inert, no advertising script is loaded on any route, and the privacy policy says the same and will be updated before that changes. No advertising will sit inside or beside a ruler face or a calibration control: a click target next to a drag-to-match outline is a bad idea on its own terms, and the instrument is the part of the page that has to be trusted.

Corrections are welcome and are treated as bugs rather than as opinions — the numbers are published with their sources so that they can be checked. Get in touch, or read the terms of use.