Pressure altitude

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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, which is controlled airspace at or above 18,000 feet).

The relation between static pressure and pressure altitude is defined in terms of the properties of the International Standard Atmosphere (ISA). Up to 36,090 ft this relation can be expressed as:

$z =\left (1-\left(\frac{P_o}{1013.25}\right)^{0.190263} \right ) \times \frac{288.15}{0.00198122}$

Where:

z = pressure altitude (feet)
$P o$ = static pressure (hPa)

[edit] Simplification

One simplification of the Pressure Altitude that is a bit more practical to pilots than the above formula is the following:

$A_{pressure}~=~A_{ASL}~+~(~29.92~inHg-P_{at~Altitude})*1000~ft/inHg.$

Where

$A p r e s s u r e =$ Pressure Altitude ( $z$ of the previous equation),
$A A S L =$ Physical Altitude above Sea Level,
$P_{at~Altitude} =$ Measured or estimated Pressure at the Altitude of interest in inches of mercury (inHg).