Online Ruler — Actual Size

A browser lays out pages in CSS pixels, not guaranteed physical millimetres. The CSS standard defines a fixed relationship between CSS units, but screen density, operating-system scaling, browser zoom, and device behavior can make a nominal CSS centimetre differ from a centimetre held against the display.

This ruler therefore starts in an explicitly approximate mode. Calibration measures how many on-screen pixels match a known physical length on your current display. Only after you confirm that match does the page label the ruler as calibrated actual size.

Use the short edge of an ID-1 format bank card or identity card. Its nominal length is 53.98 millimetres. Hold that short edge lightly against the screen and adjust the two guide lines until they meet the card corners, then select Confirm calibration.

You do not need to show or enter a name, number, photograph, chip, magnetic stripe, or any other card detail. Only the outside edge is used as a physical reference. Calibration is retained only while this page remains open.

The metric edge has a tick every millimetre. Medium ticks mark 5 millimetres, and numbered major ticks mark whole centimetres. Ten millimetres equal one centimetre.

The imperial edge divides each inch into sixteenths. Longer intermediate ticks help identify halves, quarters, and eighths, while numbered major ticks mark whole inches. One inch equals exactly 25.4 millimetres, so both scales use the same calibrated screen measurement.

Choose horizontal or vertical orientation, place one end of the object at the ruler's zero line, and drag the blue marker to the other end. The result appears at the same time in millimetres, centimetres, and inches. For keyboard control, focus the marker and use the arrow keys in one-millimetre steps; Home returns to zero and End moves to the visible limit.

Fullscreen can provide a longer working area when the browser supports it. Do not press sharp, heavy, dirty, hot, wet, or abrasive objects against a display. Measure those objects with a physical ruler or caliper instead.

Recalibrate after changing browser zoom, display scaling, screen resolution, orientation, or the monitor containing the browser window. Moving a window between monitors can change the device pixel ratio even when the viewport looks similar.

The tool watches the browser-reported screen dimensions, device pixel ratio, and orientation angle. If that environment changes after calibration, it marks the calibration stale and returns to the approximate scale. A normal window resize by itself does not invalidate a matching display calibration.

A carefully calibrated screen ruler can help with quick everyday checks such as comparing small stationery, estimating a margin, or translating a visible length between metric and imperial units. Accuracy depends on card alignment, viewing angle, the condition and tolerance of the reference card, and the construction of the display.

This is not a certified measuring instrument. Do not use it for doses or medical devices, legal evidence, manufacturing tolerances, engineering certification, parts that must fit safely, or any decision where a small error can cause harm or expense.

The page performs calibration and measurement locally. It does not request camera, screen-capture, file, location, or account access, and it does not upload a card image or measurement.

The calibrated pixels-per-millimetre value remains in page memory only. Refreshing or closing the page clears it, which also prevents a calibration from being silently reused after the display setup may have changed.