Measure anything on screen.
Pixel-perfect ruler at real physical size. Calibrate once, measure forever.
Get started
How to use the ruler.
Three steps and you're measuring at real physical size.
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Calibrate your screen
Click Calibrate in the top bar. Choose auto-detect, enter your screen diagonal, or drag a credit card to match. Your PPI is saved in the browser — you only do this once.
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Position the ruler
Snap it to the top, bottom, left, or right edge of your screen — or switch to Float mode and drag it anywhere. Use R to rotate between horizontal and vertical.
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Measure
Hold your object against the screen. The ruler shows centimetres on top and inches on the bottom simultaneously. Hover for a live readout at your cursor.
Accuracy
Why calibration matters.
Every screen has a different pixel density — a 27" 4K monitor packs 163 pixels per inch while a phone screen might hit 460 PPI. Without calibration, an online ruler assumes 96 PPI (the web standard), which can be off by 60% on a modern phone or high-DPI laptop.
Our three calibration methods cover every situation: auto-detect is instant if your device is in our database, the diagonal method works for any screen if you know its size, and the credit card method gives the highest accuracy because it uses a physical reference you're already holding.
Auto detect
±1%
Device fingerprint matched against 60+ device database. Instant, zero input.
Screen diagonal
±2%
Enter your screen size in inches. Works for any display if you know the spec.
Credit card
Best
ISO 7810 standard card is 85.60 × 53.98 mm. Physical reference, highest accuracy.
Why us
More than a simple ruler.
| Feature | Real Online Ruler | Typical ruler sites |
|---|---|---|
| Dual-scale cm + inches simultaneously | ||
| Draggable & edge-snapping ruler | ||
| Dark mode | ||
| Protractor angle measurement tool | ||
| Live measurement readout at cursor | ||
| Keyboard shortcuts | ||
| Multiple rulers simultaneously | ||
| Opacity control | ||
| Copy ruler image to clipboard | ||
| 3 calibration methods |
Shortcuts