How pressure altitude is calculated

Pressure altitude Pressure altitude is a calculation of the altitude in relation to a standardised datum (reference), in this case ISA (International Standard Atmosphere). The ISA datum is a theoretical reference level where the weight of the atmosphere is 29.92 "Hg (1013.25 mb/hPa) as measured by a barometer. Pressure altitude is the calculated height above this ISA datum based on the given pressure and elevation.