An online ruler shown at its true physical size on any screen.
One quick calibration gives you millimetre accuracy.
Drag the red handle to match the card size.
Measure any flat object with a physical ruler. Enter its length, then drag the red handle until the bar matches that object on screen.
Verify with a physical ruler: the red bar below should be exactly 50 mm.
Measure objects in cm or inches on your laptop or phone screen.
Auto Calibrate estimates your display and adjusts the ruler.
Double-tap for full screen. On Safari, add to Home Screen instead.
For best accuracy, calibrate using one of the calibration methods by pressing the recalibrate button.