Calibrate the ruler

Browser zoom must be 100%. Your setting stays in this browser.

Hold a standard bank card against the outline and adjust it until both widths match.

85.60 × 53.98 mm

Card width

Enter how many CSS pixels equal one inch. Use this if you already know the value for this display and browser zoom.

PPI means pixels per inch. The common uncalibrated browser estimate is 96.

Offline screen ruler

A self-contained ruler that keeps working without the internet.

Calibrate once for each display and keep browser zoom at 100%.
Click anywhere to add a guide line. Click a guide to remove it.
1, 2, 3 switch units.
F toggles fullscreen; D changes theme; Esc clears guides.
The ruler, interface, and saved calibration use no network connection.