On-screen Ruler

Measure on your screen in millimeters/centimeters. Calibrate via PPI or by matching a known object (e.g., a credit card is 85.6mm wide).

Calibration: match a physical card (85.6 mm)

Not calibrated: the 96 PPI starting value is an estimate. Calibrate before measuring.

Width: 0 px → 0 mm

How to calibrate

  1. Place a physical card against your screen.
  2. Drag the gray bar, use its arrow keys, or enter its pixel width until it visually matches the card width (85.6mm).
  3. Click Use card calibration. Recalibrate after changing zoom, screen or display scaling.