What Is an Online Ruler?
An online ruler is an interactive digital measuring tool that renders a 1:1 actual-size scale directly on your computer, laptop, tablet, or smartphone display. Because physical screen pixel densities (DPI) vary widely across displays, our tool includes a quick 1-click calibration step using a standard credit card (or physical ruler) so that 1 centimeter on screen corresponds to 1 actual physical centimeter.
How to Calibrate Your Screen
- Click the Calibrate Screen Size button above.
- Hold a standard credit card (or ID card) flat against your display screen.
- Drag the slider until the blue credit card graphic matches the exact physical width of your card.
- Your ruler is now 100% calibrated to a true 1:1 physical scale!