About Online Ruler

OnlineRuler.site exists to answer a surprisingly common problem: you need to measure something right now, and there's no physical ruler in reach. We built a free, accurate, browser-based ruler that turns any screen — laptop, desktop, tablet, or phone — into a precise measuring tool, with no downloads and no sign-ups.

Our Mission

Most "online rulers" assume every screen shows pixels at the same size. They don't. A 13-inch laptop and a 27-inch monitor render the same number of pixels at very different physical sizes, so an uncalibrated on-screen ruler is little more than a decoration. Our mission is to fix that — to give people a measuring tool that is genuinely true-to-scale, backed by real screen calibration rather than guesswork.

How Our Calibration Works

Accuracy comes from knowing your screen's pixel density (PPI). We offer three calibration methods so anyone can get a reliable result: automatic device detection using a database of 200+ known devices, a screen-diagonal calculation where you enter your display size in inches, and a physical reference method where you match a standard credit card (ISO/IEC 7810, 85.60mm wide) against the screen. Once calibrated, your settings are stored locally in your browser so you only do it once.

What You Can Do

Beyond a basic ruler, the tool supports measurement in millimeters, centimeters, inches, and pixels; multi-edge rulers for measuring width and height at once; an object-measurement mode for point-to-point distances; a quick unit converter; keyboard shortcuts; light and dark themes; and offline support as a Progressive Web App. We also publish companion tools like a PPI calculator and a screen-size detector.

Privacy First

We believe a simple utility shouldn't demand your personal data. Your calibration and preferences live only on your device. We don't require accounts, and we keep data collection to the minimum needed to run and improve the Service. You can read the details in our Privacy Policy.

Who Built This

OnlineRuler.site is built and maintained by a small, independent team of web engineers who care about precision, performance, and clean design. The tool is funded by unobtrusive advertising, which lets us keep it completely free for everyone.

Get in Touch

Have feedback, a feature request, or a device we should add to our calibration database? We'd love to hear from you. Visit our contact page or email hello@onlineruler.site.