True Altitude Calculator
A pressure altimeter is calibrated to the standard atmosphere, so the altitude it indicates equals true altitude only when the air column is at standard temperature. In colder than standard air the aircraft sits lower than the altimeter reads, a hazard on instrument approaches in winter. The standard correction adds roughly 4 feet for each degree Celsius of deviation from standard, per 1,000 feet above the altimeter setting source, applied with sign so cold subtracts and warm adds. This calculator takes indicated altitude, the source elevation, and outside air temperature, computes the standard temperature deviation, and returns the corrected true altitude.
True altitude formula
Standard temp = 15 - 1.98 * (indicated altitude / 1000)
Deviation = outside air temp - standard temp
Height above source = indicated altitude - source elevation
Correction = 4 * deviation * (height above source / 1000)
True altitude = indicated altitude + correction
A negative deviation (colder than standard) yields a negative correction, so true altitude is below indicated. The 4 feet per degree per 1,000 feet factor is the standard cold-temperature rule of thumb.
Temperature and altitude notes
- The altimeter reads true altitude only at standard temperature; cold air makes it over-read.
- Apply published cold-temperature corrections on approaches at designated airports in winter.
- The correction grows with both the temperature deviation and the height above the setting source.
- Use the airport elevation as the source when the altimeter setting comes from that station.
- This rule of thumb is for awareness; authoritative corrections come from the aeronautical authority's tables.
True altitude: frequently asked questions
What is true altitude?
True altitude is the actual vertical height of the aircraft above mean sea level. A pressure altimeter set to the local altimeter setting shows indicated altitude, which equals true altitude only in standard temperature conditions. When air is colder than standard, the aircraft is lower than the altimeter indicates.
How do you correct altitude for temperature?
A widely used rule of thumb adds about 4 feet of correction for every 1 degree Celsius of deviation from standard temperature, per 1,000 feet of height above the altimeter setting source. In cold air the true altitude is below indicated, so the correction is subtracted; in warm air it is added.
Why is the cold weather correction important?
When temperatures are well below standard, the altimeter over-reads, placing the aircraft lower than indicated. On an instrument approach in very cold conditions this can erode obstacle clearance, which is why pilots apply cold temperature altitude corrections at affected airports as published by the authority.
What is standard temperature at altitude?
The International Standard Atmosphere uses 15 degrees Celsius at sea level and a lapse rate of about 2 degrees Celsius per 1,000 feet. The standard temperature at a height is 15 minus 1.98 times the height in thousands of feet. The temperature deviation is the actual temperature minus this standard value.
Is this rule of thumb exact?
No. The 4 feet per degree per 1,000 feet figure is an approximation taught for quick mental correction and used in official cold-temperature tables. For precise approach corrections use the published correction values for the specific airport and altitude from the aeronautical authority, which this calculator does not replace.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.