About

About Real Sizer

Real Sizer is an on-screen ruler you calibrate in your browser. Hold a clean, flat, undamaged, smooth-edged card that conforms to the ID-1 standard just above the display without touching it. Look square-on and drag the outline until its edges match the card. Do not press or slide the card on the glass; any gap or tilt adds error outside the printed band. The match sets the ruler's physical scale, subject to that band and the calibration limits. Before calibration, the scale is assumed, not measured: CSS defines an inch as exactly 96 pixels, regardless of the display's physical size.

How it works

Read the scale and its error band together. The readout names the calibration state and shows the error band for the reference object and edge you matched. A mark labelled 10 mm is subject to that band, which combines the object's published tolerance with an allowance for lining up the edges by eye. A card's short edge gives a wider band than its long edge. When a display is too coarse for a level of graduations, such as half-millimeter marks, the ruler omits that level and reports which marks were left out.

Nothing leaves your device. There is no upload, file picker, camera access, clipboard read or account anywhere on this site. The site consists of static files, and the tools do their calculations in your browser tab. One calibration record is saved in your browser's storage for your next visit; what that record contains is listed field by field. The site's security policy also blocks connections to outside origins.

Screen calibration has limits. The methodology page explains which changes the ruler can account for and when it marks a calibration as stale. It also describes a blind spot: a move to a different monitor of the same resolution can make the scale a third wrong while leaving every browser signal unchanged. If you move to another screen, calibrate again even if the readout has not changed. The page also explains why some verified reference objects are still withheld.

Why calibration needs a physical reference

Real Sizer does not use a database of device screen sizes or try to detect display density automatically. Reported resolution does not establish physical size: Display Zoom on an iPhone can report a resolution associated with another model, a Mac's scaled resolution can describe a framebuffer rather than the panel, and privacy modes can alter these values. The screen size you enter is used only to set the outline's starting size before you match it by eye.

An ID-1 bank card is the only primary reference offered. A sheet of A4 is available for a cross-check, not a primary calibration. The data also contains fourteen other objects with their nominal dimensions, and the tool refuses them by name. Before a reference can be offered, its dimensional tolerance must be read from a published standard and recorded with at least two independent sources and a verification date. Two of the withheld objects passed that check but remain excluded because they can be mistaken for a different object. The calibration page lists all sixteen and explains each decision.

Who runs it

Real Sizer is built and run by an independent developer. The site shows no advertising. Ad slots are reserved in the written parts of the pages; they are marked in the markup and inert, and no advertising script loads on any route. No slot sits inside or beside a ruler face or a calibration control, where a click target could interfere with matching the outline.

Errors in dimensions, calculations or sources are treated as bugs. The figures are published with their sources so you can check them. Get in touch with a correction, or read the terms of use.