ISA Standard Atmosphere Calculator
This ISA standard atmosphere calculator finds the ISA temperature, pressure, density ratio and speed of sound at any pressure altitude, and your ISA deviation from the actual outside air temperature.
Valid from −2,000 ft to 60,000 ft (troposphere and lower stratosphere). Enter pressure altitude, not indicated altitude.
For training and general informational use. Use approved performance data for flight planning.
Standard atmosphere values at any altitude
The International Standard Atmosphere (ISA) is the reference used for altimeter calibration, aircraft performance charts and many exam questions. At sea level it assumes 15°C and 1013.25 hPa (29.92 inHg), with temperature falling about 2°C per 1,000 ft up to the tropopause. This calculator gives the standard values at any pressure altitude and, if you enter the actual temperature, how much warmer or colder than standard the air is.
How to use it
- Enter the pressure altitude in feet. On the ground, that is field elevation corrected for QNH.
- Optionally enter the actual outside air temperature in °C.
- Calculate to see ISA temperature, pressure, density ratio, speed of sound and, with OAT, the ISA deviation and approximate density altitude.
Method and calculation
Up to 36,089 ft: ISA temperature = 15 − 1.98 × (altitude ÷ 1,000) °C and pressure = 1013.25 × (1 − 6.8756 × 10−6 × altitude)5.2559 hPa. Above 36,089 ft, temperature is constant at −56.5°C and pressure falls exponentially. Density ratio = pressure ratio ÷ temperature ratio (in kelvin). Speed of sound = 38.97 × √(temperature in K) knots.
Worked example and practical notes
At 5,000 ft pressure altitude, ISA is 5.1°C and 843.1 hPa. On a 30°C afternoon at that level the air is ISA +24.9°C, giving a density altitude of roughly 8,000 ft, so the aircraft performs as if it were about 3,000 ft higher.
Accuracy and practical tips
Hot tropical airfields are almost always well above ISA. A field at sea level on a 34°C day is ISA +19°C, and density altitude is already over 2,000 ft. Check takeoff and climb performance charts at the real temperature, not at standard conditions.
Based on the ICAO Standard Atmosphere (ICAO Doc 7488); the same values up to 32 km are tabulated in the U.S. Standard Atmosphere, 1976. The density altitude shown uses the common 120 ft per °C approximation.
Last reviewed: October 2026
Read the guide: The International Standard Atmosphere (ISA) Explained
Frequently asked questions
What are ISA sea-level conditions?
15°C, 1013.25 hPa (29.92 inHg) and a density of 1.225 kg/m³, with temperature falling about 1.98°C per 1,000 ft.
What does ISA +20 mean?
The actual temperature is 20°C warmer than the standard temperature for that pressure altitude. Performance is reduced because the air is less dense.
Where is the tropopause in ISA?
At 36,089 ft (11,000 m). Above it the standard temperature stays at −56.5°C up to about 65,600 ft.
Is ISA the same as the weather?
No. ISA is a reference model. Real temperature and pressure differ every day, which is why pilots calculate ISA deviation and density altitude.