How to Convert Altitude to PSIA

Absolute atmospheric pressure is a measure of the pressure of the air compared to an absolute vacuum, which has an air pressure of zero. It is the sum of the pressure reading from a gauge and average barometric pressure at sea level, which is 14.696 psi (pounds per square inch). The sum is reported in pounds per square inch absolute, or PSIA.



There is a regular, known relationship between air pressure and elevation with respect to sea level. This relationship is inverse, meaning that pressure decreases as elevation increases.

Things You'll Need

  • Elevation reading
  • Calculator
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Instructions

    • 1

      Obtain the elevation of the location of interest. Elevation may be determined in a number of ways, including 3D-equipped GPS units, altimeters, and topographic maps. For example, the elevation of Denver, Colorado, is 5280 feet.

    • 2

      Convert the elevation from feet to meters, if it is not already in the metric system. Multiply by 0.3048 to perform the conversion: Denver is 5280 * 0.3408 = 1609.3 meters above sea level.

    • 3

      Multiply the elevation by 2.25577 * 10^-5 and subtract the result from 1: 1609.3 * 2.25577*10^(-5) = 0.0363, which when subtracted from 1 yields a result of 0.9637.

    • 4

      Raise the result to the exponent 5.25588 and multiply that answer by 101.325: 0.9637^5.25588 = 0.8234, multiplied by 101.325 = 84.343. This answer is in kilopascals, the SI unit of atmospheric pressure.

    • 5

      Multiply the answer in kPa by 0.145 to convert to PSIA: 84.323*0.145 = 12.227 PSIA.