1Ruler

Calibrate Your Screen Ruler

Choose the method you can verify. The result is stored locally in this browser, not in an account.

Credit card reference

Place a standard bank card over the guide and adjust the width.

85.60 mm

PPI 96.0

Screen diagonal

Enter the manufacturer’s physical diagonal for the active screen.

Diagonal in inches

Card calibration

Use the 85.60 mm width of a standard ID-1 bank card as a physical reference. Adjust the guide until both widths match, then save the resulting CSS pixels-per-inch value.

Screen diagonal

If you know the physical diagonal of the active display, 1Ruler combines it with the browser-reported screen dimensions to estimate the correct scale. This works best when the browser is on the display whose diagonal you entered.

When to recalibrate

Recalibrate after changing monitor, system scaling, browser zoom, remote-desktop settings or display mode. For legal, medical or industrial measurements, use certified equipment instead of a browser ruler.

Questions people ask before measuring

Which calibration method should I use?

Matching a standard bank card is the simplest physical reference. Screen diagonal can be useful when you know the exact active display size and are using the correct screen.

Is card calibration stored on an account?

No account is required. The saved scale stays in local browser storage on that device and can be cleared at any time.