A screen ruler that measures true physical size

The problem with most online rulers

The vast majority of “online rulers” draw every screen at 96 DPI — the pixel density of legacy CRT monitors. Modern displays range from roughly 82 PPI on a budget 27-inch monitor to over 500 PPI on flagship phones. When a site assumes 96 DPI, an “inch” is wrong by up to 5× on a phone and by 15% or more on many laptops.

That is why the whole point of this tool is calibration. Before drawing a single tick mark, ruler. measures the real pixel density of your screen using one of three methods:

• Auto-detect — matches your screen against a built-in database of iPhones, iPads, MacBooks, Android phones, laptops and common monitors by pixel resolution, then applies the factory PPI.
• Screen diagonal — you enter the diagonal from your device specs and the tool computes the PPI from your screen’s native resolution.
• Credit card — you hold a real credit, debit or ID card against a card-sized rectangle on screen and drag a slider until it fits. No extra hardware required.

Once calibrated, one centimetre on screen equals exactly one physical centimetre. Your calibration is saved in your browser’s localStorage, so the ruler is accurate on every future visit.

Built for real measuring tasks

Beyond the ruler itself, the tool includes guide lines you can drop anywhere with a click, a live crosshair with X/Y readouts in your chosen unit, and fullscreen mode for edge-to-edge measuring.

Everything runs 100% in your browser. There is no backend, no account system, no analytics and no tracking — your screen and your measurements never leave your device.

Free, private, and here to stay

ruler. is free to use with no ads and no sign-up. It is part of a small family of free tools that run entirely client-side — built to be fast, private and genuinely useful. If you have feedback or a feature request, you can get in touch.