The International Standard Atmosphere is the reference every performance chart, altimeter, and airspeed system is built around. This calculator gives the standard ISA temperature at any pressure altitude and, if you supply the actual outside air temperature, the ISA deviation that drives performance and true-airspeed corrections.
How it works
Below the tropopause the ISA temperature falls linearly from 15 degrees Celsius at sea level at a lapse rate of 1.98 degrees per 1,000 ft:
ISA temp (C) = 15 - 1.98 x (pressure altitude / 1000)This holds up to the standard tropopause at 36,089 ft, where the temperature reaches minus 56.5 degrees Celsius. Above that the model is isothermal and the temperature stays at minus 56.5 degrees. The ISA deviation is simply the actual OAT minus this standard value, expressed as ISA plus or minus a number.
Worked examples at typical flight levels
| Pressure altitude | ISA standard temp | OAT example | ISA deviation |
|---|---|---|---|
| 0 ft (sea level) | +15.0°C | +30°C | ISA+15 |
| 5,000 ft | +5.1°C | +10°C | ISA+5 |
| 10,000 ft | −4.8°C | +5°C | ISA+10 |
| 20,000 ft | −24.6°C | −30°C | ISA−5 |
| 36,089 ft (tropopause) | −56.5°C | −56.5°C | ISA 0 |
| 40,000 ft | −56.5°C | −60°C | ISA−3.5 |
These are illustrative — enter your actual OAT to get your specific deviation.
Why ISA deviation matters for performance
Aircraft performance manuals are built around ISA conditions. A positive ISA deviation (warmer than standard) means the air is less dense, which produces these effects:
- Takeoff and landing: density altitude is higher, so the required runway is longer and the climb gradient is shallower.
- Service ceiling: thinner air lowers the maximum altitude the aircraft can reach.
- Engine output: most piston and turboprop engines produce less power in warm air; turbojets are also affected but differently.
- True airspeed: for a given indicated airspeed, true airspeed is higher in warmer, less dense air.
Conversely, a negative deviation (colder than standard) means denser air and generally better performance, which is why cold winter mornings produce noticeably shorter takeoff rolls.
Pressure altitude vs. true altitude
The ISA model is defined against pressure altitude — the altitude read when the altimeter subscale is set to 1013.25 hPa (29.92 inHg). This is not the same as true altitude above mean sea level unless the local QNH happens to equal standard pressure. Use pressure altitude when feeding the ISA formula and when comparing results with performance charts, which are always based on pressure altitude.
Notes
Feed pressure altitude, not true altitude, for consistency with altimetry and performance charts. A positive ISA deviation always means degraded performance and a higher true airspeed for a given indicated airspeed, which is exactly why hot days call for careful runway and climb planning.