Screen Ruler (px/cm/inch)

An on-screen ruler you calibrate, so it is actually accurate.

Step 1 — calibrate

Hold a bank card, driving licence or any ID-1 card against the screen and drag the slider until the outline matches its width exactly.

85.60 mm

Step 2 — measure

012345678910

Numbers are centimetres in millimetre mode and inches in inch mode. Zooming the page invalidates the calibration, so reset the zoom to 100% before measuring.

Result

CHECK

Calibrate against a bank card first

A browser has no way to know the physical size of your screen. The CSS specification fixes an inch at 96 CSS pixels whatever the display actually is, which is why an uncalibrated on-screen ruler can be badly wrong on a high-density laptop and worse on a phone. Matching the outline above against a bank card solves it, because the ID-1 card format is 85.60 mm wide everywhere in the world. Recalibrate if you move the window to a different monitor or change the browser zoom, since both change the relationship this measurement depends on.

Diagnostic telemetry
Calibration
not calibrated yet
Pixels per millimetre
3.785
Pixels per inch (CSS)
96.1
Approximate physical PPICSS pixels per inch multiplied by the device pixel ratio
96
Browser assumptionFixed by specification, and unrelated to your actual screen
96 CSS px per inch
Error if uncalibratedHow far out a ruler assuming 96 px per inch would be on this display
0%

What this cannot tell you

  • A browser cannot know the physical size of your screen. The ruler is only as accurate as the calibration you set against a card of known size.
  • Changing the browser zoom, or moving the window to a different monitor, invalidates the calibration. Reset zoom to 100% and recalibrate after either.
  • For anything where a millimetre matters, use a physical rule. This is for quick measurements, not for engineering.

Take this with you

Screen Ruler (px/cm/inch) on RunTheTests — free browser-based repair tools, no sign-up.