Engineering
Barometric Altitude Pressure & ISA Tropospheric Lapse Rate formulas
Compute International Standard Atmosphere (ISA) air pressure P(h) = 101.325 * (1 - 2.25577E-5 * h)^5.25588 in kPa and standard temperature from 0 to 11,000 meters.
Use a fixed standard-atmosphere tropospheric approximation
This dry-air reference model uses sea-level pressure 101.325 kPa, sea-level temperature 15°C and lapse rate 0.0065 K/m. Here h is geopotential altitude in meters, restricted to 0–11000 m. Geometric GPS elevation is not exactly the same quantity.
The coefficients are rounded approximations. This model is not current weather, a calibrated altimeter, density altitude or an aircraft performance calculation. It does not account for humidity or local pressure and temperature variations.
The example locks B2 and B4 to its standard reference values. Changing temperature alone while retaining the fixed pressure coefficient would not create a self-consistent alternative atmosphere. A different lapse-rate model must recompute its temperature ratio and exponent.
| Elevation Benchmark | Altitude h (B3) | ISA Pressure P(h) (kPa) | ISA Temp T(h) (°C) |
|---|---|---|---|
| Mean Sea Level (MSL) | 0 m | 101.325 kPa (100% P0) | +15.0 °C |
| Denver / High Plateau | 1,600 m | 83.53 kPa (~82.4% P0) | +4.6 °C |
| 5,000 m Base Camp | 5,000 m | 54.02 kPa (~53.3% P0) | -17.5 °C |
| Jet Cruise (10,000 m) | 10,000 m | 26.44 kPa (~26.1% P0) | -50.0 °C |
ISA Barometric Air Pressure P(h) in kilopascals (kPa)
The exponent is approximately g0/(R_air*L), using consistent SI constants. Pressure is returned in kPa; do not read it as hPa or pascals.
=IF(COUNT(B2:B4)<>3,"Invalid inputs",IF(OR(B2<>101.325,B4<>15,B3<0,B3>11000),"Invalid inputs",ROUND(B2 * ((1 - 0.0000225577 * B3) ^ 5.25588), 2)))ISA Standard Outside Air Temperature T(h) = 15 - 0.0065 * h_meters
Applies the standard 6.5 °C per 1,000 m tropospheric lapse rate up to 11,000 m.
=IF(COUNT(B2:B4)<>3,"Invalid inputs",IF(OR(B2<>101.325,B4<>15,B3<0,B3>11000),"Invalid inputs",ROUND(B4 - 0.0065 * B3, 2)))Try the example
Use the original values or change them to test your understanding.