Online ruler

Online ruler: what it is for

The ruler appears at actual size after a quick calibration: hold a standard bank card (3.37 in / 85.6 mm) or a US quarter (0.955 in / 24.26 mm) against the screen and adjust until the drawing is the same size. The calibration is saved in your browser.

The ruler shows inches with 1/16 divisions or centimeters with millimeter marks, and you can tap it to mark and read a measurement.

How to use

1. Choose to calibrate with a card or a coin.
2. Hold the object against the screen and adjust the size of the drawing.
3. Use the ruler and tap it to mark the measurement.

When to use it

• Measuring small objects with no ruler around.
• Checking the size of a screw, ring or part.
• Measuring in centimeters for purchases from abroad.

Frequently asked questions

Why do I need to calibrate?

Every screen has a different pixel density, and the browser does not report the real physical size. By comparing with an object of known size, such as an 85.6 mm (3.37 in) card, the ruler starts showing real inches and centimeters.

What object should I use to calibrate?

Any card in the standard bank card size (credit, debit, gift card, driver's license) is 85.6 × 53.98 mm (3.37 × 2.125 in). You can also use a US quarter, which is 24.26 mm (0.955 in) across.

Can I measure something bigger than the screen?

Yes: measure it in parts. Mark where the ruler ends, slide the object and add up the pieces. For long measurements, a tape measure is still the most accurate.

Is the measurement exact?

Once calibrated, the error is usually under 1 mm on phones. Hold the object flat against the screen and look straight at it to avoid parallax error.

Is it really free?

Yes, 100% free. No sign-up, no subscription and no usage limits. All Tools123 tools are free and run right in your browser.

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