Altitude-Pressure Calculator

Calculate atmospheric pressure based on altitude above sea level and vice versa. Values are based on the U.S. Standard Atmosphere, which covers altitudes from -609.6 to 84852 meters (-2000 to 278386 feet). Pressures and altitudes calculated are typical values and do not account for variations due to weather.

In this form, input two of the three values. The calculator will calculate the missing value.

Note: standard sea-level pressure is 1 atmosphere, and 1 millibar is equal to 1 hectopascal.

Sea-level pressure: pascals bars millibars millimeters of mercury kilopascals inches of mercury pounds per square inch atmospheres

Altitude: feet miles meters kilometers

Pressure: pascals bars millibars millimeters of mercury kilopascals inches of mercury pounds per square inch atmospheres
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In the atmosphere of Earth, air pressure decreases as altitude increases. The average pressure at sea level is 1 atmosphere (1013.25 millibars), varying slightly with weather.

The pressure recorded by weather stations, and seen on weather reports, is the atmospheric pressure adjusted to sea level. For example, the mountain resort town of Big Bear Lake, California is at an altitude of 2070 meters (6791 feet). If the weather report shows the pressure in Big Bear Lake as 1012 millibars, the actual pressure at that location is about 787 millibars.

As air pressure decreases, the boiling point of water also decreases. At the altitude of Big Bear Lake, water boils at 93.1°C (199.6°F), less than the sea-level boiling point of 100°C (212°F). At the Armstrong limit of about 19200 meters (63000 feet), water boils at normal human body temperature of 37°C (98°F), meaning that exposed body fluids (such as saliva, tears, and the water in the lungs) boil. At about 34500 meters (113000 feet), water's boiling point is equal to its melting point: 0.01°C (0.018°F). This temperature and pressure is referred to as the triple point of water. At pressures below the triple point, liquid water cannot exist. Note: the formula used to calculate boiling point has a small error of a fraction of a degree C/F. As a result, this calculator gives the triple point of water as about 0.00598 atm, when it is actually 0.00604 atm.