About Real Screen Ruler
Real Screen Ruler is a free, browser-based measuring tool that works on any device — desktop, laptop, tablet, or phone. No downloads, no sign-up, no ads. Just a ruler that works.
What Is This Tool?
Real Screen Ruler is a screen ruler that displays a calibrated measuring scale directly in your web browser. You can measure in centimeters, millimeters, or inches, place guide lines to mark measurements, and display horizontal and vertical rulers simultaneously.
The tool is designed to be as accurate as a physical ruler after calibration. Before calibration, any online ruler is just a guess — because browsers don't know how big your screen is physically. After calibration, the tick marks correspond to real-world distances.
Why We Built It
We needed a quick-access ruler for everyday tasks — sizing images, checking object dimensions without hunting for a tape measure, helping with remote instructions that reference physical sizes. The existing tools were cluttered with ads, required Flash (or old JavaScript), or simply didn't calibrate properly on modern high-DPI displays.
We built Real Screen Ruler to be fast, clean, and genuinely accurate. The ruler renders using the HTML5 Canvas API with device pixel ratio awareness, so tick marks are sharp on retina and HiDPI screens. The three calibration methods cover every scenario from auto-detecting your device to using a physical credit card as a reference.
Accuracy and Calibration
The accuracy of any screen ruler depends entirely on calibration. Here's what you need to know:
- Without calibration, accuracy may be off by 10–50% depending on your screen
- After auto-detect calibration, accuracy is typically ±1–2% for recognized devices
- After credit card calibration, accuracy is typically ±1%, limited by visual alignment
- Fine-tuning with the PPI ± buttons can bring accuracy within ±0.5% on a well-calibrated screen
Real Screen Ruler is suitable for everyday practical measurements — cutting fabric, sizing prints, checking package dimensions, and similar tasks. For critical precision applications, use a calibrated physical instrument.
Privacy
We do not collect any personal data. Your calibration settings are stored locally in your browser's localStorage — they never leave your device. We don't use tracking cookies, analytics scripts, or advertising networks. See our Privacy Policy for full details.
Technical Details
The ruler is built with Astro and Tailwind CSS, rendering via the HTML5 Canvas API. Everything runs client-side — there's no server computation involved in measurements. The device detection database covers 25+ common devices including iPhones, iPads, MacBooks, popular Android phones, and common monitors.
Calibration is stored under the key ror_state in localStorage, including PPI value, unit preference, and guide line positions. Dark mode preference is stored under ror_dark.
Contact and Feedback
We're always interested in improving the tool. If you find a device the auto-detect doesn't recognize, or notice measurements seem off, please reach out.