Online Ruler

A free on-screen ruler showing pixels, centimeters, and inches. Calibrate to your monitor by matching a real credit card on the screen.

1. Calibrate your monitor

Place a real ID or credit card (85.6 mm wide) flat on the screen and adjust the slider until the orange box matches its width.

PPI: 96

85.6 mm — credit card width

2. Use the ruler

Units

Length

Why calibrate?

Browsers report a fixed CSS resolution (96 DPI) regardless of your real screen. Without calibration, "1 cm" on the ruler may not match a physical centimeter.

Saved

Your calibration is saved to this browser, so you only have to do it once per device.