Actual-size measurement guide

Calibrate an online ruler for actual size

An online ruler becomes dependable after its screen scale matches your physical display. Use a known object, keep the zoom fixed, and save the calibration on the screen you will measure with.

Open calibration tool Calculate PPI

Best calibration methods

Set browser zoom to 100%

Use the browser default zoom before calibrating. Browser zoom changes CSS pixel size, so a saved calibration should be used at the same zoom level.

Choose a dependable reference

A standard credit card is the easiest reference because its ISO ID-1 width is 85.60 mm. Larger objects such as Letter or A4 paper can also work when they fit on screen.

Save the scale on the same display

Calibrate on the monitor, laptop screen, tablet, or phone you plan to measure with. Recalibrate after moving the browser to a different screen.

Reference object sizes

Use flat, undamaged objects with published dimensions. The longer the reference span, the easier it is to spot small scale errors.

Object Actual size When to use it
Credit card / ID-1 card 85.60 mm x 53.98 mm Best all-purpose reference for actual-size calibration.
US Letter paper 8.5 in x 11 in Useful for larger desktop monitors when a full sheet fits on screen.
A4 paper 210 mm x 297 mm Useful outside the US and for checking a longer metric span.
US dollar bill 6.14 in x 2.61 in Handy when a card is unavailable, but verify the bill is not folded or worn.

Fix common accuracy problems

Measurements changed after zooming: reset browser zoom to 100% and calibrate again.

Measurements are correct on one display but wrong on another: each display needs its own calibration.

Phone measurements are cramped: rotate the phone to landscape and recalibrate in that orientation.

Screen diagonal math is close but not exact: use credit card calibration to account for browser and display scaling.

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