Station Pressure to Pressure Altitude Calculator

Station pressure is the absolute atmospheric pressure measured by the barometer at its actual elevation, before any reduction to sea level. It is the correct input for pressure altitude because the standard atmosphere relation PA = 44330 (1 - (Ps/1013.25)^0.1903) maps a pressure directly onto the height in the ISA model at which that pressure occurs. Working from station pressure avoids the intermediate step of undoing a sea level reduction, which matters because reduction formulas assume a fictitious temperature profile in the imaginary layer between the station and sea level, and that assumption is the largest error source at high elevation sites. Sensitivity is easy to remember: near sea level each hPa is about 8.3 m or 27 ft of pressure altitude, so a barometer carrying 1 hPa of drift moves the answer by roughly 27 ft, and typical digital station barometers hold 0.3 to 1 hPa. This page also derives sea level pressure and the equivalent altimeter setting from the entered station elevation and temperature, and adds density altitude using the 120 ft per degree C ISA deviation rule. Two limitations deserve attention. The formula assumes the ISA lapse rate of 6.5 degrees C per km continues unbroken to the surface, so the output is an idealized mapping rather than a physical height above ground, and it should not be applied above about 11 km where the standard atmosphere becomes isothermal and a different expression governs.

Reducing barometric readings to sea level became necessary as soon as national weather services started drawing synoptic charts in the 1860s and 1870s, because raw station pressures from sites at different elevations cannot meaningfully be contoured. Reduction formulas derived from Laplace's hypsometric equation were adopted, and the choice of assumed mean column temperature was debated for decades, since that choice determines how much a mountain station's pressure gets inflated on the map. Aviation added a second family of reductions and with them the international Q code. QFE, the pressure at the aerodrome reference point, makes an altimeter read zero on the runway; QNH, derived so the altimeter reads field elevation on the ground, became the standard clearance value; and QNE refers to flying on the standard 1013.25 hPa setting. These conventions were consolidated by the International Civil Aviation Organization after its founding in 1944 and codified alongside the ICAO Standard Atmosphere of 1952. In routine practice today, METAR reports carry the altimeter setting in the A group in inches of mercury across North America or in the Q group in hectopascals elsewhere, automated surface observing systems retain raw station pressure internally, and aircraft flight management computers invert the standard atmosphere relation continuously from static port pressure.