Free Online Screen Ruler
Measure inches, centimeters, and millimeters in real physical size. Calibrate once, measure anything.
Real‑Size Screen Ruler – Measure Anything on Your Display
Our free online screen ruler uses DPI calibration to display true physical inches, centimeters, and millimeters directly on your screen. After calibrating (using a credit card), you can measure real objects against your monitor, tablet, or laptop screen.
Properly calibrated, the ruler matches real‑world dimensions.
Horizontal, vertical, and L‑shape – measure width and height at once.
Click, drag, and release to draw lines that show distance in inches and cm.
Generate a PDF with a calibration square and a printable ruler.
No signup, no uploads – everything stays in your browser.
Your DPI is saved in localStorage, so you calibrate once per device.
How to Use the Online Screen Ruler
1. Calibrate first. The default DPI (96) may not match your monitor. Click "Calibrate" and follow the instructions: drag the overlay until a physical credit card matches the green rectangle. Or use "Auto‑detect" for an estimate.
2. Select ruler mode and units. Choose Horizontal, Vertical, or L‑shape. Pick inches, centimeters, or both.
3. Draw measurement lines. Click "Draw Lines", then click and drag anywhere on the canvas. Each line shows length in inches and centimeters.
4. Print a ruler. Click "Printable PDF" to generate a page with a 1‑inch calibration square and a ruler you can cut out.
Understanding DPI and Calibration
DPI (dots per inch) determines how many pixels fit into a physical inch. The auto‑detection uses your browser’s `devicePixelRatio`, but for pixel‑perfect accuracy, manual calibration with a credit card (85.6mm × 53.98mm) is recommended.
Frequently Asked Questions
Learn how to use this tool correctly by reading our step-by-step tutorial.
