Pressure altitude

Given an atmospheric pressure measurement, the pressure altitude is the imputed altitude that the International Standard Atmosphere (ISA) model predicts to have the same pressure as the observed value.

The National Oceanic and Atmospheric Administration (NOAA) published the following formula^[1] for directly converting atmospheric pressure in millibars (mb) to pressure altitude in feet (ft): $${\displaystyle h=145366.45\left[1-\left({\frac {\text{Station pressure in millibars}}{1013.25}}\right)^{0.190284}\right].}$$

In aviation, pressure altitude is the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.921 inches of mercury (1,013.2 mbar; 14.696 psi) as measured by a barometer.^[2] It indicates altitude obtained when an altimeter is set to an agreed baseline pressure under certain circumstances in which the aircraft’s altimeter would be unable to give a useful altitude readout. Examples would be landing at a high altitude or near sea level under conditions of exceptionally high air pressure. Old altimeters were typically limited to displaying the altitude when set between 950 mb and 1030 mb. Standard pressure, the baseline used universally, is 1013.25 hectopascals (hPa), which is equivalent to 1013.25 mb or 29.92 inches of mercury (inHg). This setting is equivalent to the atmospheric pressure at mean sea level (MSL) in the ISA. Pressure altitude is primarily used in aircraft-performance calculations and in high-altitude flight (i.e., above the transition altitude).