Enter the altitude in feet and the actual temperature reading into the calculator to determine the ISA temperature difference.

ISA Temperature Formula

The following formula is used to calculate an ISA temperature difference. 

ISA T =  ST - AT
  • Where ISA T is the temperature difference between the actual temperature and the standard temperature
  • AT is the actual temperature reading (C)
  • ST is the standard temperature at that altitude

To calculate ISA temperature, subtract the actual temperature from the standard temperature.

The standard temperature can then be calculated from the following formula:

ST = 15 – 1.98*A/1000

  • Where ST is the standard temperature
  • A is the altitude (ft)

What is ISA Temperature?

An ISA temperature is a measure of the difference between the actual temperature reading on an aircraft and the standard temperature at that altitude.

How to claculate ISA Temperature?

The following example problem outlines the steps and information needed to calculate an ISA temperature.

First, determine the actual temperature. This is the temperature read by the aircraft. For this example, the actual temperature is -30C.

Next, determine the altitude of the aircraft. In this example, the altitude is 26,000 feet.

Next, determine the standard temperature at that altitude using the formula above:

ST = 15 – 1.98*A/1000

ST = 15 – 1.98*26000/1000

ST = -36.48 C

Finally, calculate the ISA temperature difference using the formula above:

ISA T = ST – AT

ISA T = -36.48C – (-30) C

ISA T = – 6.48 C