A little ruler, wherever you are.
Set the screen scale with a standard bank card, then place a small object beside the left ruler edge. Drag A and B to its endpoints. The large reading shows the distance between them.
Calibrate before measuring
Different screens display the same webpage at different physical sizes. We cannot detect that size accurately. Match the calibration guide to the short edge of a standard bank card (53.98 mm), then save. Check the scale against your card again before an important measurement.
Cards with unusual dimensions will give an incorrect scale. The displayed decimals help position the markers; they are not a claim of measurement accuracy. Use a physical ruler when accuracy matters.
Keep your screen scale the same
Calibration is saved only in this browser. Rotating the screen, changing the window width, browser zoom, display scaling, or monitor can require calibration again. Pinch zoom pauses measurements until you return to normal zoom. Displays with identical reported settings cannot always be distinguished, so check the guide after moving to another monitor.
Keyboard controls
Tab to marker A or B, then use the arrow keys to move it by 1 mm after calibration. Hold Shift for 10 mm. Home and End move to the ends of the ruler. The markers can cross; the reading is always the distance between them.
More tools: Bubble level · Compass