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Actual Size Online Ruler (Calibrated to Your Screen)

A ruler drawn to your screen. Calibrate it once with any bank or ID card, then measure small objects in inches or centimeters.

1. Calibrate with a card

Hold any bank card, ID or library card (85.6 × 54 mm) against the screen and move the slider until the outline matches its edges.

Match a real card

2. Measure

Place the object at the zero mark. Tap or drag on the ruler to read a length.

Marked length0
Scale100%

How it works

Screens have different pixel densities, so a CSS inch is rarely a real inch. Matching a card (85.6 × 53.98 mm, ISO/IEC 7810 ID-1) corrects for your display.

Your calibration is saved in this browser only.

Questions people ask

Is an online ruler accurate?

After calibration with a card it is usually within about 1 mm over 10 cm, limited by how precisely you match the card edges.

Does it work on a phone?

Yes, but you can only measure as far as the screen is wide.

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