Online ruler › How to Calibrate an Online Ruler

How to Calibrate an Online Ruler

Last updated: October 25, 2026 · By the TrueRuler team

To calibrate an online ruler, set browser zoom to 100%, then match the on-screen reference to a real object such as a bank card. Calibration tells the ruler how many pixels make one inch on your particular screen, called PPI. Without it, a line drawn as 10 cm can measure anywhere from 6.8 cm to 10.5 cm.

Step by step

Set your browser zoom to 100% (Ctrl/Cmd + 0).
Open the online ruler and check the status dot. Green means your device was recognized and no calibration is needed.
If the dot is not green, choose a method: enter your screen diagonal, or open the full options and match a bank card, A4 or US Letter sheet, or any length you can measure.
For the card method, hold a bank card flat against the blue bar and drag the slider until the bar matches the card's 85.60 mm width. Press Save calibration.
Check the result: show a 5 cm bar with the Show an exact size box and hold a physical ruler against it.

Which method is best?

Method	Accuracy depends on	Best for
Bank card (85.60 × 53.98 mm, ISO/IEC 7810 ID-1)	How carefully you align the card	Any screen
Screen diagonal	Whether the listed diagonal is exact	A quick start
Device detection	Whether your panel matches the database entry	Phones and common laptops
A4 / US Letter / custom length	The reference you use	When you have no card

How is PPI calculated?

PPI equals the diagonal length in pixels divided by the diagonal size in inches. A 15.6 inch laptop with a 1920×1080 screen has a diagonal of √(1920² + 1080²) ≈ 2,202.9 pixels, so 2,202.9 ÷ 15.6 ≈ 141.2 PPI. One inch is then 141.2 pixels and one millimeter about 5.56 pixels.

Common mistakes

Browser zoom is not 100%. Zoom changes the size of a CSS pixel.
Display scaling changed. Recalibrate after changing Windows or macOS scaling.
A new monitor. A calibration only fits the screen it was made on.

Open the online ruler