Online Ruler

What it is

Online Ruler is a small, independent set of measuring tools that run entirely in your web browser. The main tool is an actual-size screen ruler with centimeter, millimeter and inch scales. The site also offers an image measurement tool for reading lengths and angles off a photo once you mark one known distance, and a length unit converter for millimeters, centimeters, meters, inches, feet and screen pixels.

The tools are free, need no account and install nothing. Nothing you measure is sent anywhere: the ruler, the converter and the photo tool all do their arithmetic on your device. The ruler works around the limitation that browsers cannot ask a display how large it physically is by requiring calibration with a bank card known to follow ISO/IEC 7810 ID-1 standard dimensions, or by entering your visible screen diagonal. The ruler saves the resulting CSS-pixel mapping for your browser, display and zoom level.

Questions from the bench

How accurate is this online ruler?

Accuracy depends on the current screen mapping, endpoint placement and viewing angle. After calibrating with a compliant ISO/IEC 7810 ID-1 bank card, check its 85.60 mm edge on the ruler to verify your setup. Browsers cannot read physical display size, so the accuracy page explains the error sources.

Do I have to calibrate every time?

The mapping is saved in your browser, but you should check it when you return. The ruler recommends a review after detectable zoom, DPR or screen changes. Recalibrate after changing display or scaling, and verify physically because different displays can report the same browser values.

Why should I calibrate with a bank card?

A bank card confirmed to follow ISO/IEC 7810 ID-1 has standardized 85.60 × 53.98 mm edges, so it serves as a convenient reference. Not every payment or identity card follows ID-1, so you should confirm the format first.

Does the ruler work on phones and tablets?

Yes. The ruler supports touch dragging. Its first-frame default is a top ruler for desktop and tablet, or a left ruler when a coarse pointer and a viewport short side below 600 CSS px indicate a phone-sized layout. You can change the visible rulers.

Is anything uploaded?

No. Calibration and preferences are stored in your browser's localStorage; measurements never leave your device. The privacy policy provides full details about stored data.