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Rulers

Html is a mishmash of nonsense units. It even uses the names of metric units with alternative definitions. Read this page and cry

pixels: -

Device pixel ratio: -

Resolution: -

Calibrated scale (including dpr) = -

Nominal Millimeter

This ruler has the correct size if your computer, monitor, browser combo respect the metric system. Desktop computers with full HD monitors usually do so in rough approximation.

Millimeter scaled with inverse dpr

Scale your desktop browser (ctrl+scrollwheel) and this ruler will stay the same size.

Millimeter scaled with dpr

This will typically be approcimately accurate on high resolution displays as well as normal resolution displays, if you don't zoom. It violates the definition of the millimeter to do so.

Calibrator tool

Use mouse scrollwheel or arrows to calibrate you monitor with an ISO 7810 card.

⬆ ⬇

Calibrated ruler

After calibration (above) this ruler has the right size. It violates the definition of the millimeter to do so.