Air Pressure at Altitude Calculator
Enter an altitude to instantly find the air pressure at that height using the ICAO International Standard Atmosphere model. The calculator gives you pressure in multiple units, the ISA air temperature, air density, and a pressure-vs-altitude chart from sea level to your chosen height. Switch to Custom mode to override the sea-level pressure and temperature with your own measured values.
How air pressure changes with altitude
Atmospheric pressure is the weight of the column of air above a given point. As altitude increases, there is less air above, so pressure falls. The relationship is not linear: pressure drops rapidly in the lower troposphere (about 1.2 hPa per 10 m near sea level) and more slowly at higher altitudes. At the summit of Mont Blanc (4,808 m) pressure is about 55% of sea level; at the top of Mount Everest (8,848 m) it falls to roughly 33%. This drop in pressure reduces the partial pressure of oxygen, which is why climbers need supplemental oxygen above about 7,000 m.
ISA standard atmosphere and the barometric formula
The International Standard Atmosphere (ISA), defined by ICAO and ISO 2533, divides the atmosphere into layers with different temperature lapse rates. In the troposphere (sea level to 11 km), temperature decreases at 6.5 C per 1,000 m. At 11 km (the tropopause) temperature stabilises at -56.5 C and stays constant up to 20 km. In the first stratospheric layer (20-32 km) it warms at 1 C per 1,000 m. Each layer uses a different form of the barometric formula: gradient layers use a power-law expression (P = P0 x (1 + L*h/T0)^(g*M/(R*L))), while isothermal layers use an exponential decay (P = P0 x exp(-g*M*h/(R*T))). The custom mode applies the simpler single-layer exponential formula, suitable when you have a measured reference pressure and a known temperature.
Practical applications
Atmospheric pressure at altitude matters in many fields. Aviation uses pressure altitude to calibrate altimeters and to set standard QNH/QFE settings. Mountaineers and physiologists use it to calculate the partial pressure of oxygen and assess altitude sickness risk. Meteorologists and weather modelers need accurate pressure profiles for forecasting. Engineers design pressure vessels, aircraft cabins, and life-support systems around these values. Cooks at altitude notice that water boils at a lower temperature (about 90 C at 3,000 m) because the lower air pressure reduces the boiling point.
Air density and its effects
Air density (kg/m3) decreases with both altitude and temperature. It is calculated from the ideal gas law: density = pressure / (R_specific x temperature in Kelvin), where R_specific is 287.05 J/(kg*K) for dry air. At sea level ISA conditions, density is 1.225 kg/m3; at 5,500 m it is about half that. Lower density reduces lift (aircraft need a longer runway), engine power (naturally aspirated engines lose about 3% power per 300 m), and the cooling effectiveness of air. Turbocharged and supercharged engines compensate by forcing more air mass into the cylinders.
ISA Standard Atmosphere: Pressure and Temperature by Altitude
| Altitude (m) | Altitude (ft) | Pressure (hPa) | Pressure (atm) | Temperature (C) | Air density (kg/m3) |
|---|---|---|---|---|---|
| 0 | 0 | 1013.25 | 1.0000 | 15 | 1.2250 |
| 500 | 1640 | 954.61 | 0.9421 | 11.75 | 1.1673 |
| 1000 | 3281 | 898.76 | 0.8870 | 8.5 | 1.1117 |
| 1500 | 4921 | 845.6 | 0.8348 | 5.25 | 1.0581 |
| 2000 | 6562 | 795.01 | 0.7847 | 2 | 1.0066 |
| 3000 | 9843 | 701.21 | 0.6920 | -4.5 | 0.9093 |
| 4000 | 13123 | 616.6 | 0.6085 | -11 | 0.8194 |
| 5000 | 16404 | 540.48 | 0.5334 | -17.5 | 0.7364 |
| 6000 | 19685 | 472.17 | 0.4661 | -24 | 0.6601 |
| 8000 | 26247 | 356.51 | 0.3519 | -37 | 0.5258 |
| 10000 | 32808 | 265 | 0.2615 | -50 | 0.4135 |
| 11000 | 36089 | 226.32 | 0.2234 | -56.5 | 0.3639 |
| 15000 | 49213 | 121.11 | 0.1195 | -56.5 | 0.1948 |
| 20000 | 65617 | 54.75 | 0.0540 | -56.5 | 0.0880 |
| 30000 | 98425 | 11.97 | 0.0118 | -46.5 | 0.0184 |
Values from ICAO International Standard Atmosphere (ICAO Doc 7488, 1993). Pressure in hPa, temperature in C.
Frequently asked questions
What is standard sea-level air pressure?
The International Standard Atmosphere defines sea-level pressure as exactly 101,325 Pa, equal to 1 atmosphere (atm), 1013.25 hPa (or mbar), 760 mmHg (Torr), or 29.92 inHg. Real weather stations measure pressures that vary around this by roughly plus or minus 5 hPa.
How much does pressure drop per metre of altitude?
Near sea level the drop is roughly 12 Pa per metre (or about 1 hPa per 8.5 m). The rate decreases with altitude: at 5,500 m the same pressure drop requires about 16 m of climb, and at 11,000 m it takes about 20 m. As a rule of thumb, pressure halves for every 5,500 m (18,000 ft) of altitude gain.
What is the difference between ISA mode and custom mode?
ISA mode uses the ICAO 1976 Standard Atmosphere, which defines a fixed sea-level pressure of 1013.25 hPa and a temperature lapse rate of 6.5 C per 1,000 m. It is accurate for standard conditions and used in aviation altimetry. Custom mode applies the simpler single-layer barometric formula with your measured reference pressure and temperature, which is more accurate for actual weather conditions when you have station data.
Why does the boiling point of water decrease at altitude?
Water boils when its vapor pressure equals the surrounding atmospheric pressure. At altitude the atmospheric pressure is lower, so water reaches that equilibrium at a lower temperature. At 3,000 m (about 9,843 ft) water boils near 90 C instead of 100 C, meaning cooking times for boiling food must be increased by roughly 10-15%.
What is the air pressure in an airplane cabin?
Modern aircraft cabins are pressurized to an equivalent altitude of about 1,800 to 2,400 m (6,000 to 8,000 ft), giving a cabin pressure of roughly 750 to 810 hPa. This is a compromise between passenger comfort and the structural weight needed to maintain true sea-level pressure at cruising altitudes above 10,000 m.
What is pressure altitude and why does it matter in aviation?
Pressure altitude is the altitude in the ISA at which a given pressure is found, assuming standard sea-level conditions. Altimeters measure pressure and convert it to altitude using the standard atmosphere. When the actual sea-level pressure differs from 1013.25 hPa, pilots adjust the altimeter subscale (QNH) so it reads true altitude. Density altitude is pressure altitude corrected for temperature, and it determines aircraft performance.