Online ruler
A calibrated on-screen ruler in inches, centimeters, and millimeters. Screen sizes and browser zoom vary, so calibrate first: the credit card method is the most accurate.
We can't read your exact physical screen pixels from the browser, so this suggests a calibration based on your device's published specs. For guaranteed accuracy, use Credit Card calibration.
Calibration complete
Measure objects against the ruler edge
Drag the ruler to align its zero point with the edge of your object.
Test your calibration
Pick a length below, then hold a physical ruler or tape measure up to the bar. If it's off, recalibrate: Credit Card is usually the fix.
About this online ruler
This is a virtual ruler that runs entirely in your browser: an on-screen ruler you can hold real objects up to instead of reaching for a physical one. It works as an actual-size ruler once calibrated, showing inches, centimeters, and millimeters at true screen scale, roughly equivalent to a 12-inch ruler stretched across a laptop display or a shorter span on a phone.
No screen ruler can read your monitor's exact physical pixel density from the browser, so every calibration method here is an estimate. Credit Card calibration is the most accurate because it's checked against a real, fixed-size object in front of you, rather than a published spec that may not match your actual display after OS or browser zoom.