📏 Ruler

Display size—

Resolution—

Zoom—

Calibrate to your screen

Your display's diagonal is printed on its box or in the model name (e.g. a 24″ monitor, a 14.6″ tablet). One number calibrates exactly.

Display size: diagonal

Hold a standard bank/ID card flat against the screen and drag the slider until the on-screen card is exactly the same width, then Save.

85.6 × 53.98 mm

Fine-tune with a real ruler

The site's cm mark measures cm on a real ruler

Why does a ruler need calibrating?

A screen only knows its resolution (e.g. 1920×1080), not its physical size — a 24″ monitor and a 6″ phone can share the same resolution. Browsers assume a fixed 96 pixels per inch, which is only right for some displays. Setting the display's real size (or matching a card standardized worldwide, ISO 7810 ID-1) pins the ruler to real millimetres.