ISA Temperature Deviation Calculator
The International Standard Atmosphere (ISA) defines a reference temperature at every altitude, starting at 15 degrees Celsius at sea level and falling at a fixed lapse rate. Comparing the actual outside air temperature against this standard gives the ISA deviation, a number that drives true airspeed corrections, density altitude, and performance planning. Enter your pressure altitude and the actual temperature to get the standard temperature and your ISA deviation.
ISA deviation formula
ISA temp (C) = 15 - lapse * (altitude / 1,000) [up to 36,089 ft]
Above 36,089 ft: ISA temp = -56.5 C (held constant)
ISA deviation = actual OAT - ISA temp
The default lapse rate of 1.98 degrees Celsius per 1,000 feet is the ICAO standard 6.5 degrees per kilometre. A positive deviation means warmer than standard (ISA+), a negative one means colder (ISA-).
Worked example
- At 10,000 ft, ISA temperature = 15 - 1.98 * 10 = minus 4.80 degrees Celsius.
- If the actual OAT is 0 degrees, ISA deviation = 0 - (minus 4.80) = ISA+4.80.
- That warmer-than-standard air raises density altitude and true airspeed.
ISA deviation: frequently asked questions
What is the ISA temperature at altitude?
The International Standard Atmosphere has a sea-level temperature of 15 degrees Celsius and a lapse rate of about 1.98 degrees Celsius per 1,000 feet (6.5 degrees per kilometre) up to the tropopause near 36,089 feet. ISA temperature = 15 minus 1.98 times altitude in thousands of feet.
What is ISA deviation?
ISA deviation, often written ISA+ or ISA-, is the difference between the actual outside air temperature and the standard ISA temperature for that altitude. ISA dev = actual temperature minus ISA temperature. A warm day at altitude might be ISA+15, meaning 15 degrees warmer than standard.
Why does ISA deviation matter?
Warmer than standard air is less dense, which reduces engine and wing performance, increases true airspeed for a given indicated airspeed, and raises density altitude. Performance charts and true airspeed corrections are often expressed relative to ISA, so the deviation is a key input.
Does the standard lapse rate continue above the tropopause?
No. In the standard atmosphere the temperature stops decreasing at the tropopause (about 36,089 feet) and holds at about minus 56.5 degrees Celsius through the lower stratosphere. This calculator applies the constant tropopause temperature above that altitude.
Official sources
- NOAA, NASA, USAF: U.S. Standard Atmosphere, 1976.
- FAA: Pilot's Handbook of Aeronautical Knowledge (standard atmosphere).
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.