Online Ruler is a free, browser-based measuring tool. It turns your phone, tablet, or computer screen into an accurate ruler — no download, no installation, and no account required.
The tool works by scaling on-screen tick marks to match your device's real pixel density (PPI), so that a centimeter, inch, millimeter, or pixel shown on screen corresponds to the same real-world measurement. Because screen sizes and pixel densities vary widely between devices, we offer several calibration methods — automatic detection, manual device selection, screen diagonal entry, and credit-card-based calibration — so you can get an accurate reading regardless of what device you're using.
This project is independently built and maintained. We don't require sign-ups, don't sell your data, and keep the tool free to use.