How to use this tool
- 1Tap 'Calibrate' and hold a bank or ID card against the on-screen box.
- 2Drag the slider until the box matches your card exactly.
- 3Tap 'Done'. The ruler is now accurate for your screen.
- 4Place a small object along the ruler and read the centimeters and millimeters.
About this test
Screens report their size in CSS pixels, which do not map to real centimeters on phones, so an uncalibrated on-screen ruler is always slightly off. Calibrating against a standard 85.6 mm card fixes the scale to your exact display.
Once calibrated, it is handy for quick measurements of small items when a physical ruler is not around. For objects longer than the screen, scroll the ruler sideways.
Privacy: this tool runs entirely in your browser. No image, audio or device data is recorded or sent to any server.
Frequently asked questions
Why do I need to calibrate?
Because every screen has a different pixel density. Matching a real card once makes the centimeters physically correct on your specific phone.
How accurate is it?
After calibration it is accurate to a millimeter or two, enough for everyday measurements, though not a replacement for a precision instrument.