ISA Deviation Calculator

Free ISA deviation calculator for pilots. Enter a flight level or altitude and the outside air temperature to get the ISA temperature and the deviation (ISA +/−), or load an airport's METAR to get it at the field.

ISA temperature

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Deviation

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Quick reference

ISA is 15 °C at sea level, about 5 °C at 5,000 ft, −5 °C at 10,000 ft, −25 °C at FL200, −45 °C at FL300 and −56.5 °C from FL361 up. A quick rule: ISA = 15 − 2 × (altitude in thousands of feet).

Planning the wind too? Try the crosswind calculator.

Frequently asked questions

›How is the ISA temperature calculated?

The International Standard Atmosphere starts at 15 °C at sea level and cools by 6.5 °C per kilometre (about 1.98 °C per 1,000 ft) up to the tropopause at 36,089 ft. Above it the temperature stays at −56.5 °C. So at FL350 ISA is about −54.3 °C.

›What does ISA +10 mean?

The air is 10 °C warmer than the standard atmosphere at that pressure altitude. Warmer air is less dense, so engines make less thrust, take-off and climb performance drop and true airspeed is higher for the same indicated airspeed. Performance charts usually take the ISA deviation as an input.

›Should I use pressure altitude or altitude?

Pressure altitude. In flight that is the flight level. At an airport, the calculator works it out from the elevation and the QNH in the METAR (about 27–30 ft per hPa below 1013).

›How accurate is it?

The formula is the ICAO standard (Doc 7488) and is exact up to about 65,000 ft. The result is for planning and training. Always use your aircraft's performance data and your operator's procedures.

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