Calculate ISA deviation, density altitude, and reverse-solve altitude or temperature from altitude, altimeter setting, and actual temperature.

Atmosphere model estimate. Use approved aircraft performance data for flight planning.

Model range: −610 to 20,000 m (−2,001 to 65,617 ft).

ISA Temperature Formula

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

ISA T = AT - ST
  • 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

ISA temperature is the standard temperature at the entered altitude. To calculate ISA deviation, subtract the standard temperature from the actual temperature; positive deviation means warmer than standard.

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

ST = 15 - 1.9812*A/1000

This temperature model uses pressure/geopotential altitude from −610 to 20,000 m. The lapse rate applies through 11,000 m; above that and through 20,000 m, ST is −56.5 °C. The reverse altitude mode solves within the troposphere only. Density altitude uses the approximate formulas PA (ft) = field elevation (ft) + (29.92 − altimeter setting inHg) × 1,000 and DA (ft) = PA + 120 × (actual °C − ISA °C). Use approved aircraft performance data for flight planning.

  • 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 calculate 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 -30 °C.

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 - 1.9812*A/1000

ST = 15 - 1.9812*26000/1000

ST = -36.5112 °C

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

ISA T = AT - ST

ISA T = (-30) °C - (-36.5112) °C

ISA T = +6.5112 °C