In aviation, pressure altitude is the indicated altitude when an altimeter is set to 1013 hPa (29.92 inHg US and Canada). It is primarily used in aircraft performance calculations, and in high-altitude flight (Class A airspace in the United States).
The relation between static pressure and pressure altitude is defined in terms of the properties of the Internatinal Standard Atmosphere (ISA). Up to 36,090 ft this relation can be expressed as:
z =(1-(Po/1013.25)^0.190263)*288.15/0.00198122
Where:
z = pressure altitude (feet)
Po = static pressure (hPa)
See also: