Is it possible to get a users physical screen size without the use of a physical object or the diagonal measurement?

Question

Question says it all really. I want to be able to display something at actual size, so, for example, if something is displayed in the browser as 20cm, I want to be able to measu

Answer 1

You can't do this with absolute certainty, no. For example, if the display device is not a screen, but a video projector, the effective size will depend on the distance of the projector to the wall, which can change without the operation system getting any feedback.

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Most operation systems have some dpi setting (dots per inch), which might either be configured by the user or system administrator, hard-coded (if the video device is build in and can't be changed, like a laptop screen), or maybe automatically discovered (if the video protocol supports this).

Either this setting or a similar browser setting will be used by the browser to calculate absolute sizes used in CSS, like cm, mm, in, pt, and so on.

So you could simply rely on the browser to show it right, or tell your user to set the setting right. (Of course, on a video projector with projection size 3×4 m you don't really want to show a 20 cm object.)