Air pressure at altitude from the standard atmosphere, with temperature, air density and the boiling point of water, or the altitude for a given pressure.
At 1,500 m (4,921 ft) the standard atmosphere gives 845.6 hPa — 83.5% of sea-level pressure — and 5.3 °C. Water boils at 95.0 °C there.
This height is in the troposphere (−6.5 K/km).
g₀ = 9.80665 m/s², M = 0.0289644 kg/mol, R* = 8.31432 J/(mol·K) (1976 standard).
IAPWS-IF97 saturation-temperature equation (eq. 31); it reproduces the IAPWS check value Ts(0.1 MPa) = 372.755919 K.
Pressure at a height comes from the barometric formula of the U.S. Standard Atmosphere 1976: 1013.25 hPa and 15 °C at sea level, temperature falling 6.5 °C per kilometre up to 11 km, then steady at −56.5 °C to 20 km. The boiling point of water at that pressure comes from the IAPWS-IF97 saturation equation. Start from a pressure instead and the same formula runs backwards to the altitude.
Cooks, climbers, pilots and engineers use it to see how thin the air is. The default, 1,500 m, gives 845.6 hPa, 83.5% of sea-level pressure, and water boiling at 95.0 °C; on the summit of Everest (8,849 m) water boils at 70.2 °C.
The standard atmosphere is an average: real pressure moves with the weather, and the air temperature shown is the model's, not today's. Enter the day's sea-level pressure under More options for a weather-adjusted figure.
Checked against: U.S. Standard Atmosphere 1976 table (geometric altitude): 1000 m → 8.9876×10⁴ Pa, 281.651 K, 1.1117 kg/m³
Checked against: U.S. Standard Atmosphere 1976 table: 11 000 m geometric → 2.2700×10⁴ Pa
Checked against: U.S. Standard Atmosphere 1976 table: 20 000 m → 5.5293×10³ Pa, 216.65 K
Checked against: Python decimal: inverse troposphere formula gives geopotential 5574.44 m (the ICAO 500 hPa level, 5574 m), converted to geometric height 5579.33 m
1013.25 hPa, the same as 101.325 kPa, 1 atm, 760 mmHg or 29.92 inHg, at an air temperature of 15 °C. These are the sea-level values of the U.S. Standard Atmosphere 1976 and of the ICAO standard atmosphere used in aviation. Actual sea-level pressure rises and falls with the weather around this value.
About 1 °C lower for every 300 m of height near sea level. At standard pressure water boils at 99.97 °C at sea level, 94.6 °C in Denver (1,609 m), 87.8 °C in La Paz (3,640 m) and 70.2 °C on Everest (8,849 m), by the IAPWS-IF97 saturation equation. Food simmered in water cooks more slowly at altitude because the water cannot get hotter than this.
Near sea level it falls by about 12 hPa per 100 m, or 36 hPa (1.07 inHg) over the first 1,000 ft, which is why pilots use a rule of 1 inHg per 1,000 ft. The drop slows with height: pressure halves at about 5.5 km (500 hPa at 5,579 m) and is under a quarter of sea level above 11 km.
The share of oxygen stays at 20.9% of dry air, but each breath holds fewer molecules because the total pressure is lower. The oxygen partial pressure, which drives how much reaches the blood, scales with the pressure ratio: 83.5% of the sea-level value at 1,500 m, 64% in La Paz at 3,640 m, and 31% on the summit of Everest in the standard atmosphere.
Weather reports give pressure reduced to sea level (QNH or mean sea-level pressure) so that maps compare like with like. The actual station pressure in Denver is near 834 hPa in the standard atmosphere. Enter the reported sea-level figure in the Sea-level pressure option to get the station pressure and boiling point for today's weather.
Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 6 worked examples whose answers come from independent sources; for example, “1,000 m” is checked against U.S. Standard Atmosphere 1976 table (geometric altitude): 1000 m → 8.9876×10⁴ Pa, 281.651 K, 1.1117 kg/m³.
U.S. Standard Atmosphere, 1976 (NOAA/NASA/USAF), NASA-TM-X-74335; IAPWS R7-97(2012) — IAPWS-IF97, region 4 saturation-temperature equation.
6 worked examples with independently sourced answers ship with this calculator. They run in the test suite; you can run them here too.
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