Formula Used
P = P₀ × (T₀ / T)gM/(RL)
Isothermal layer:
P = P₀ × e−gMh/(RT)
P represents atmospheric pressure at altitude. P₀ represents reference pressure. Temperature and altitude must use compatible absolute units.
The ISA option applies separate lapse rates by atmospheric layer. The custom model applies one lapse rate. The isothermal option keeps temperature constant.
Air density uses dry-air and water-vapour partial pressures. Humidity therefore changes density estimates. Virtual temperature can also adjust pressure calculations.
How to Use the Calculator
- Select the required calculation direction.
- Enter altitude, pressure, or density-altitude conditions.
- Choose input and output units.
- Select an atmospheric model and altitude reference.
- Keep standard defaults or enter local weather data.
- Set the table range, interval, and display precision.
- Press the calculation button to view detailed results.
Worked Examples
| Example | Altitude | Approximate standard pressure | Typical use |
|---|---|---|---|
| Sea level | 0 m | 101.325 kPa | Reference atmosphere |
| Low mountain | 1,000 m | About 89.9 kPa | Travel and weather |
| High mountain | 5,000 m | About 54.0 kPa | Altitude physiology |
| Commercial flight | 10,000 m | About 26.4 kPa | Aviation comparison |
| Aircraft cruise | 30,000 ft | About 30.1 kPa | Cabin-pressure studies |
| Mount Everest | 8,848.86 m | About 31.4 kPa | Extreme-altitude comparison |
Frequently Asked Questions
Why does atmospheric pressure decrease with altitude?
Less air remains above higher locations. The overlying air therefore exerts less weight. Pressure consequently falls as altitude increases.
What is the International Standard Atmosphere?
It is a reference atmospheric model. It defines standard pressure and temperature profiles. Real weather can differ significantly.
What is pressure altitude?
Pressure altitude is altitude within the standard atmosphere matching measured pressure. Pilots use it for performance calculations. It differs from true geometric elevation.
What is density altitude?
Density altitude is standard-atmosphere altitude matching actual air density. Temperature and humidity affect it. Higher density altitude generally reduces aircraft performance.
Can the calculator handle locations below sea level?
Yes, negative altitude values are accepted. The model extrapolates the lower troposphere. Very deep values receive a warning.
What is station pressure?
Station pressure is measured at the station elevation. It is not corrected to sea level. Weather reports may provide corrected sea-level pressure instead.
Why do geometric and geopotential altitude differ?
Geometric altitude measures physical distance from a reference surface. Geopotential altitude accounts for changing gravity. Their difference grows at high altitudes.
Does humidity change atmospheric pressure?
Humidity primarily changes air density and virtual temperature. Its pressure effect depends on the model. This calculator offers optional virtual-temperature correction.
Which pressure unit should I choose?
Use the unit required by your application. Aviation often uses hectopascals or inches of mercury. Engineering commonly uses pascals or kilopascals.
Why can calculated pressure differ from weather observations?
Standard models assume idealised atmospheric conditions. Actual temperature, humidity, and weather systems vary. Local measurements are therefore more accurate.
What altitude range does the ISA calculation support?
The layered implementation reaches 84,852 geopotential metres. Values above that limit are extrapolated. A visible warning identifies such results.