About Screen Ruler
Screen Ruler is a free tool that turns your browser window into a real, physically accurate ruler — in inches and centimeters — so you can measure objects by holding them up to your screen.
Why calibration is necessary
Browsers only know about pixels — they have no reliable way to know the physical size of your monitor or phone screen. Two screens with the exact same resolution can be completely different sizes in the real world, so a ruler that assumes a fixed 96-pixels-per-inch, like most "online ruler" pages do, will be wrong on most modern displays.
Screen Ruler solves this with a short, one-time calibration: you hold a real object of known size — a credit card, a dollar bill, or a sheet of paper — up to your screen and drag a slider until an on-screen shape matches it exactly. That gives an exact pixels-per-millimeter value for your specific display, which the ruler then uses to draw accurate inch and centimeter markings.
Your calibration is stored only in your browser, using localStorage. It is never uploaded, and Screen Ruler has no backend server to send it to even if it wanted to.
Who built this
Screen Ruler is an independent, single-page project — no company, no funding, no team. The source is public on GitHub, maintained by MD-Sharik.
How it's built
The entire tool is a single static HTML file — no framework, no build step, no analytics, no third-party scripts beyond loading the Inter typeface from Google Fonts. The ruler itself is drawn on an HTML canvas at your device's actual pixel ratio, so the lines stay crisp regardless of screen resolution.