True Altitude Temperature Calculator
Your altimeter only reads true altitude when the atmosphere matches the International Standard Atmosphere (ISA) model. When the outside air temperature (OAT) deviates from ISA, the aircraft is at a different altitude than the altimeter shows. The classic rule is "high to low or hot to cold, look out below": in cold air, the true altitude is lower than indicated, creating a terrain clearance risk. This calculator applies the standard FAA/ICAO temperature correction formula to compute true altitude. Enter the indicated altitude (corrected for altimeter setting), the outside air temperature in Celsius at that altitude, and the ISA temperature is calculated automatically. Cold weather temperature corrections on instrument approaches are required by ICAO Annex 2 when temperatures are below 0 degrees C.
True altitude temperature correction formula
ISA Temp (C) = 15 - (Indicated Altitude / 1,000 x 1.98)
True Altitude = Indicated Altitude x (1 + (OAT - ISA Temp) / 273)
The 273 in the denominator converts Celsius to Kelvin (0 C = 273 K). This formula is from the FAA Aeronautical Information Manual. The correction is proportional to both the altitude and the temperature deviation. At 10,000 ft with OAT 25 degrees C below ISA, the error is approximately 916 ft (the true altitude is 916 ft lower than indicated).
Cold temperature altimetry
- Below 0 degrees C, cold temperature corrections must be applied to instrument approach altitudes (ICAO Annex 2, FAA AIM Chapter 7).
- The rule of thumb: for every 10 degrees C below ISA, altitude error is approximately 4% per 1,000 ft (or 40 ft per 1,000 ft per 10 degrees C cold).
- Cold temperature tables are published in the FAA Chart Supplement for affected airports.
- Modern FMS systems may automatically apply cold temperature corrections; verify your aircraft's capability.
True altitude calculator: frequently asked questions
What is true altitude?
True altitude is the actual height of an aircraft above mean sea level (MSL). The altimeter displays pressure altitude adjusted for altimeter setting, but when the air temperature deviates from the International Standard Atmosphere (ISA), the altimeter reading is inaccurate. True altitude corrects for this.
Why does temperature affect altimeter accuracy?
Altimeters measure pressure and infer altitude using a standard atmosphere model (ISA). When air is colder than standard, it is denser, so pressure levels are closer together and the aircraft is actually lower than the altimeter indicates. Colder than ISA means true altitude is lower than indicated.
What is the ISA temperature?
The International Standard Atmosphere (ISA) defines sea-level temperature as 15 degrees C with a lapse rate of 1.98 degrees C per 1,000 ft. At any altitude, ISA temperature = 15 - (altitude in 1,000 ft x 1.98). At 10,000 ft ISA, temperature is -4.8 degrees C.
When is the temperature correction most important?
Temperature correction is critical in cold weather operations, particularly on instrument approaches. ICAO Annex 2 and FAA guidance require pilots to apply cold temperature corrections to approach altitudes when temperatures fall below 0 degrees C to ensure terrain and obstacle clearance.
What formula does this calculator use?
True Altitude = Indicated Altitude + (Indicated Altitude x (OAT - ISA Temp) / 273). This is the standard FAA/ICAO temperature correction formula from the Aeronautical Information Manual. OAT is the outside air temperature in Celsius at the indicated altitude.
Official sources
- FAA Aeronautical Information Manual (AIM), Section 7-2-3: faa.gov.
- ICAO Annex 2: Rules of the Air: icao.int.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.