Keep Browser Zoom At 100%.
Zoom changes how CSS pixels are mapped to the display. Calibrate and measure at 100% zoom when exact size matters.
accuracy guide
Make Screen Measurements More Reliable.
An online ruler can be useful for quick measurements, but physical accuracy depends on calibration, screen density, browser zoom, and display scaling.
Use A Physical Reference.
Credit card calibration is practical because it compares the ruler to a real object with a standard width of 85.60 mm.
Recalibrate When Displays Change.
External monitors, display scaling, and operating-system settings can change the physical result. Save a fresh calibration for each screen.