Online Ruler
A real-size ruler on your screen. Calibrate it once with a credit card or paper, then hold an object against the screen or drag the two markers to measure.
Every screen has a different pixel density, so a ruler is only exact after calibration. Lay a reference object flat on the screen and drag the slider until the blue rectangle has exactly the same width as the object.
2. Ruler
3. Measure between the markers
Drag the two orange markers on the ruler to measure the distance between them.
Vertical ruler
Tips for an accurate measurement
Calibrate with the biggest reference object that fits your screen: a sheet of A4 or Letter paper gives a more exact result than a credit card because any error is spread over a longer length. Use browser zoom 100% – if you change the zoom, calibrate again. A screen can show a few millimeters of error at the edges, so measure near the center for small objects.
Questions
How can an online ruler be accurate?
A browser does not know the real pixel density of your screen, so it cannot guess true size. This ruler asks you to match the on-screen rectangle to an object of known size (a credit card is always 85.6 mm wide). After that, every millimeter is scaled correctly for your screen.
Which reference object should I use?
Any of the listed objects works. The larger the object, the more accurate the calibration. A sheet of A4 or US Letter paper is the most precise; a bank card is the easiest.
Do I have to calibrate again?
Your calibration is saved in this browser. Calibrate again if you use another screen, change the browser zoom or the display resolution, or plug in an external monitor.
Can I measure on my phone?
Yes. Calibrate with a credit card and hold the object against the screen. The ruler works in portrait and landscape; for a long object use the vertical ruler or the markers.
Is anything uploaded?
No. Everything runs in your browser; only your calibration number is saved on your device.