I've always wondered how we get from 29.92 to a 1013.25 baseline myself.
29.92 is inches of mercury, whereas 1013.25 is mbar (obsolete unit).
1 bar = pressure exerted by a force of 1 kilogram-force over a unit surface area of 1 square centimeter.
The official pressure unit is now the Pascal : 1 Newton over 1 square meter, hence 1 bar = 100 000 Pascal, 1mbar (millibar) = 100 Pascal aka 1 hPa (hectopascal)