Online Ruler
Measure real-world distances on your screen. Because screens vary in pixel density, the ruler must be calibrated first: hold a bank card against the outline below and adjust the slider until they match, then the rulers are accurate to within a millimetre.
How to Calibrate
Every card of the type used for banking and ID follows the same standard, so the outline below is exactly 85.60 mm wide. Place a real card against your screen and move the slider until the outline matches it. That single measurement sets the scale for both rulers.
- Calibrate against a standard bank card
- Centimetre and inch rulers to true scale
- Tick marks at 1 mm, 5 mm and 10 mm
- Shows the resulting pixels per inch
- Calibration is remembered for the session
Why calibration is needed
A CSS pixel is not a fixed physical size. On a high-density display a CSS pixel may be a third of a millimetre, while on an older monitor it could be more than a third of a millimetre again. The browser does not reliably report the physical size of your display, so a known reference object is the only dependable way to establish the true scale.
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