Online Ruler

A free, accurate ruler for your screen. Supports centimetres and inches. Calibrate to your actual screen for precise measurements.

○ estimated

96 dpi

Calibrate for accuracy

How to use the online ruler

Select your unit — centimetres or inches — using the toggle.
Hover your cursor over the ruler to measure.
Click Calibrate and use a credit card for accuracy.

Why calibrate your screen?

Every monitor has a different pixel density. Without calibration the ruler uses 96 DPI — fine for most screens but off by up to 20% on high-density displays. Calibrating takes 10 seconds and gives you sub-millimetre accuracy.

Frequently asked questions

Is this ruler accurate?: After calibration, accurate to within 0.5 mm on most screens.
Does it work on mobile?: Yes. On mobile the ruler switches to vertical mode on the left side of the screen. Hold your phone beside an object to measure it.

Other tools

Vertical ruler → Dedicated vertical ruler for desktop.
Online protractor → Measure angles from 0° to 360°.