Zellio.io

Online Ruler (cm & inches)

Measure on screen with a calibrated centimeter and inch ruler.

Processed locally — nothing is uploaded

About On-Screen Ruler

Accuracy is bounded by the display itself — good to about a millimetre, which suits a print size or a component but not a machining tolerance. Browser zoom rescales the page, so any change to it invalidates the scale until you set it again.

Measure small objects on screen in centimetres or inches with a ruler you calibrate once against a credit card, so the scale matches your display. It runs entirely in your browser.

Measure physical objects on your screen in centimetres or inches using a ruler you calibrate once against a known object, such as a credit card. After calibration the on-screen scale matches real-world size on your specific display.

It is handy when there is no physical ruler nearby — sizing a small item, checking a print dimension, or judging proportions in a design before committing to them.

Learn how this works

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Frequently asked questions

How accurate is an on-screen ruler?

Only as accurate as its calibration. Screens vary widely in pixel density, so a ruler assuming a standard DPI can be off by a noticeable margin. Calibrate against a known object — a bank card is 85.6 mm wide — before relying on it.

Why does it read differently on my phone and laptop?

Because those screens have very different pixel densities and the browser's idea of a CSS inch is nominal, not physical. Each device needs calibrating separately.

Can I measure something bigger than my screen?

Mark the object at the screen's edge, slide it along, and add the measurements. Accuracy degrades with each repositioning, so for anything much larger a physical tape is the better tool.

Common Use Cases

  • •Measuring something small when no physical ruler is nearby
  • •Checking a print size or margin on screen
  • •Comparing an object against centimetre and inch scales at once

More On-Screen Ruler tools

On-Screen Ruler

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