ISA temperature and deviation calculator
The International Standard Atmosphere defines one temperature for every altitude. Comparing the real temperature against it gives a single number, written as ISA +10 or ISA -15, that performance tables and cruise charts are built around.
Calculator
This is an educational and planning aid. Use current official weather, your aircraft flight manual, and the rules that apply to your flight before you act on any figure here. See the aviation disclaimer.
What the result means
The standard atmosphere starts at 15 C at sea level and cools by 1.98 C for every 1,000 ft, until it reaches the tropopause at 36,089 ft where it levels off at -56.5 C. The deviation is simply your measured temperature minus that standard figure.
A positive deviation means warmer than standard, so the air is thinner, true airspeed is higher than indicated for a given altitude, and the aircraft climbs worse. A negative deviation means colder and denser, which helps performance but can cause the altimeter to over read.
When this is useful
- When a cruise performance table is indexed by ISA deviation rather than by raw temperature.
- When checking whether a cold weather altimeter correction is needed, which matters on an approach in genuinely cold air.
- To sanity check an outside air temperature reading in the cruise, since a wild deviation usually means a faulty probe or a wrong altitude.
How it is calculated
Below the tropopause, standard temperature is 15 C minus 1.98 C for each 1,000 ft of pressure altitude. Above 36,089 ft the standard atmosphere holds temperature constant at -56.5 C, so the calculator stops applying the lapse rate there.
The deviation is the measured temperature minus the standard temperature, and the calculator also writes it in the shorthand form used on charts.
ISA temperature = 15 - (1.98 x altitude in thousands of feet), down to -56.5 C at the tropopause
Worked example
- Pressure altitude
- 10,000 ft
- Observed temperature
- -15 C
Standard temperature at 10,000 ft is -4.8 C. The observed -15 C is 10.2 C colder, so conditions are ISA -10.
Check: Standard atmosphere tables list -4.8 C at 10,000 ft, which matches. The lapse rate is often rounded to 2 C per 1,000 ft in the cockpit, giving -5 C, close enough for mental arithmetic.
Limitations
- The standard atmosphere is an agreed reference, not a forecast. Real temperature profiles include inversions and layers that no single lapse rate describes.
- Above the tropopause the model holds temperature constant, which is a simplification. Real stratospheric temperature varies with latitude and season.
- Use pressure altitude rather than indicated altitude, otherwise the standard temperature is taken from the wrong level.
Common mistakes
- Confusing standard temperature with standard pressure. They are separate parts of the same model, and setting 1013.25 on the subscale does nothing to the temperature.
- Reporting a deviation with the wrong sign. Colder than standard is a negative deviation, written ISA minus.
- Applying a deviation worked out at cruise altitude to a performance figure for a different altitude.
Sources
- U.S. Standard Atmosphere, 1976 (NASA-TM-X-74335) , NASA, NOAA and US Air Force
- Earth Atmosphere Model , NASA Glenn Research Center