Barometric Pressure Altitude Calculator
Air pressure falls predictably with height, so a pressure reading can be turned into an altitude. This tool uses the international barometric formula based on the ICAO standard atmosphere: a sea-level pressure of 1,013.25 hPa, a temperature of 15 degrees Celsius, and a lapse rate of 6.5 degrees per kilometre. Enter the measured pressure and, if you have it, the local sea-level pressure to estimate height above the reference. The result is the standard pressure altitude that aviation, weather, and survey instruments use. Both the reference pressure and the measured pressure are user-editable so you can set a local altimeter setting.
Barometric altitude formula
altitude (m) = 44,330 * (1 - (P / P0) ^ (1 / 5.255))
altitude (ft) = altitude (m) * 3.28084
P is the measured pressure and P0 the reference sea-level pressure. The constants come from the ICAO standard atmosphere: 44,330 metres is the scale factor and the exponent 1 / 5.255 derives from the lapse rate, gravity, and the gas constant for dry air. The formula assumes the standard temperature profile, so real altitudes differ slightly when the air column is warmer or colder than standard.
Worked example
A station reports 900 hPa against a standard sea-level reference of 1,013.25 hPa. The ratio is 900 / 1,013.25 = 0.8882. Raised to the power 1 / 5.255 gives 0.97771. So altitude = 44,330 * (1 - 0.97771) = 988.65 metres, which is 3,243.59 feet. This matches the roughly 1,000 metre height where pressure falls near 900 hPa in the standard atmosphere.
Frequently asked questions
What is the international barometric formula?
It is a closed-form relationship between pressure and height derived from the ICAO standard atmosphere within the troposphere, where temperature falls at a constant 6.5 degrees Celsius per kilometre. It gives the standard pressure altitude: the height in a standard atmosphere at which the measured pressure would occur. Aviation altimeters work on the same principle.
Why does the result differ from my GPS altitude?
The formula assumes the standard temperature profile and a fixed sea-level pressure. On a warmer-than-standard day the real air column is taller, so true altitude exceeds pressure altitude, and the reverse on a cold day. GPS measures geometric height directly and so will differ. For precise work, correct for actual temperature and the local altimeter setting.
What reference pressure should I enter?
For standard pressure altitude, leave the reference at 1,013.25 hPa, the ICAO sea-level standard. To find height above your local field, enter the current local sea-level pressure (the altimeter setting or QNH) as the reference, and the result becomes height above that datum.
Does this work above the troposphere?
The constant-lapse-rate formula is valid up to about 11,000 metres (the tropopause). Above that, in the stratosphere, temperature stops falling and a different relationship applies, so results from this calculator become less accurate at very low pressures.
Official sources
- NOAA National Weather Service: pressure and the standard atmosphere.
- U.S. Standard Atmosphere, NOAA / NASA / USAF: U.S. Standard Atmosphere 1976.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.