About Online Ruler

What this site is

Online Ruler is a small, independent set of measuring tools that run entirely in your web browser:

• an actual-size screen ruler with centimeter, millimeter and inch scales;
• an image measurement tool for reading lengths and angles off a photo once you mark one known distance;
• 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.

Why it exists

A browser cannot ask a display how large it physically is. It only knows the resolution in pixels. Most "actual size" rulers on the web therefore assume a generic pixel density and are visibly wrong on a phone, a high-DPI laptop or a large monitor.

We built this ruler around a calibration step instead. You match an on-screen reference to a bank card known to follow ISO/IEC 7810 ID-1 (85.60 × 53.98 mm), or enter your visible screen diagonal. The ruler saves the resulting CSS-pixel mapping for this browser, display and zoom; saving it is not itself proof of physical accuracy. The full derivation is on the methodology page, and the ways this can still go wrong are described on the accuracy page.

How the tools work

Everything is ordinary web technology: the ruler is an SVG drawn at pixel positions computed from your calibrated pixel density, the photo tool uses a canvas, and the converter uses the exact definition 1 in = 25.4 mm. The only data the site keeps is your calibration value and a few preferences, stored in your browser's local storage so they survive a reload. You can clear them at any time from the calibration dialog or your browser settings. Our privacy policy lists everything the site and its third parties may store.

Content principles

The guides and help text on this site follow a few rules:

1. No invented precision. We do not publish accuracy percentages we have not measured. Where a limit exists, we explain how to check it yourself.
2. Standards over hearsay. Dimensions come from published standards (ISO/IEC 7810 for cards, ISO 216 for paper, the CSS reference pixel) and the arithmetic is shown so you can verify it.
3. Tools first. Every guide links to the tool it explains, and every tool page links back to the explanation.
4. No fake social proof. The site does not display ratings, reviews or testimonials it has not actually collected.

Articles use the site organization as their publisher and do not claim an individual expert reviewer. Corrections and reproducible discrepancies can be sent through the contact page.

Advertising

The site may show advertisements served by Google AdSense to cover hosting costs. Ads are placed below the tools and never overlap the measuring area. The cookie policy explains what the ad network may store.

Corrections

If a dimension, formula or claim on this site is wrong, tell us via the contact page. Corrections are applied to the page and its "last updated" date is changed.