Calibrated Screen Ruler
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Calibrate a ruler against a known physical length and measure small objects on your screen. Recalibrate after display or zoom changes.
Preparing your tool…
How to use it
- Choose a known physical reference, such as the width of a standard bank card, or enter your own measured reference.
- Adjust the on-screen reference until its width matches the real object, then explicitly confirm calibration.
- Place a small object against the zero line and move the marker to its end. Recalibrate after zoom, rotation or a display change.
Example: A standard bank card is used as an 85.60 mm wide calibration reference. It is not scanned or photographed; no card information is entered.
Details & limits
Manual reference: 5–300 mm. Fits only the visible measuring area. The page invalidates detected view changes but cannot identify every monitor change. No precision guarantee or saved calibration.
Can a website know the real size of my screen automatically?
Not reliably. Browser pixels do not establish physical millimetres. Calibration links a known real length to this particular display view. Alignment, reference accuracy and screen changes affect the result. Use it for approximate everyday measurements, not safety-critical or certified work.