How to convert to px?

Answer 1

In general you cannot rely on a fixed pixel size for fonts, the user may be scaling the screen and the defaults are not always the same (depends on DPI settings of the screen etc.).

Maybe have a look at this (pixel to point) and this link.

But of course you can set the font size to px, so that you do know how many pixels the font actually is. This may help if you really need a fixed layout, but this practice reduces accessibility of your web site.