How are the font sizes calculated, anyway?

[xiaweitang]

We talk about 12pt, 14pt font sizes. Do those numbers represent physical size in some unit or number of pixels? I guess it is the former, because same font size looks different for different DPI values, but in what unit? Also a relevant question: does browsers like firefox and chromium use different system of font sizes, because to get the same physical size, I alwasy need to set larger values in firefox than in other applications. Setting dpi and resolution values using about:config has no effect.

[GNU-Fan]

You are right., there is another variable.
The dpi sets the ratio between the 'virtual' pixel size and the actual size on the physical screen.
Ideally, you would set one global dpi value which is respected by every application.

But as it's the case so often in GNU/Linux, there exist several approaches in parallel
X Server DPI

[linxt]

We talk about 12pt, 14pt font sizes. Do those numbers represent physical size in some unit or number of pixels? I guess it is the former, because same font size looks different for different DPI values, but in what unit? Also a relevant question: does browsers like firefox and chromium use different system of font sizes, because to get the same physical size, I alwasy need to set larger values in firefox than in other applications. Setting dpi and resolution values using about:config has no effect.

As to the first part of your post, points are a physical measure originally used by printers and publishers to represent different sizes of fonts (called type back then). Don't know what system it is in, imperial, metric, etc.