It will also vary, often inaccurately from typeface to typeface. 12pt is supposed to be 12pt (6 lines per inch) and going between two differing typefaces, you

It will also vary, often inaccurately from typeface to typeface. 12pt is supposed to be 12pt (6 lines per inch) and going between two differing typefaces, you may get upwards of a 50% variation. That doesn't even account for interface scaling to physical size, which no major OS does right, and only phone devices tend to come close.