Free tool

Density altitude calculator

Enter an altimeter reading, a QNH and a temperature, and this calculator gives you the density altitude with the arithmetic that produced it.

Pressure altitude

1,054

Feet. What the altimeter would read with 1013 on the subscale, which is the height the performance tables are written against.

ISA deviation

+30.1

Degrees warmer or colder than the standard atmosphere at that pressure altitude, which here is 12.9 degrees.

Density altitude

4,664

Feet. The altitude at which the standard atmosphere has the density this air actually has.

The working

  1. Pressure altitude. The subscale is 1004 hPa and standard is 1013.25 hPa, so the standard datum is 9 hPa below the one set. Pressure altitude = 1054 ft.
  2. The exam shortcut, 27 ft per hPa, gives 1050 ft. The figure above uses the full standard atmosphere relation, which is why the two differ slightly.
  3. ISA temperature at 1054 ft = 15 - 1.98 x 1.1 = 12.9 degrees.
  4. ISA deviation = 43 - 12.9 = 30.1 degrees.
  5. Density altitude = pressure altitude + 120 x ISA deviation = 1054 + 120 x 30.1 = 4664 ft.

what this does to the takeoff

The aeroplane behaves as if it were at 4,664 ft, not 1,054 ft.

Density altitude is the single number that says how the aeroplane will perform, because thin air takes something from every part of the takeoff at once. The engine draws in less mass, so it makes less power. The propeller pushes less mass, so it makes less thrust. The wing meets less mass, so it needs a higher true airspeed to produce the same lift, while the airspeed indicator still reads the same number.

The result is a longer ground run, a slower acceleration to a rotate speed that is now a higher true airspeed, and a reduced rate of climb once airborne. Obstacle clearance after takeoff suffers for the same reason.

Exam trap. Density altitude is not a height you fly at, and it is not read off an instrument. Nothing in the cockpit displays it. It is worked out from pressure altitude and temperature, and a question that offers you an altimeter reading as the density altitude is offering you the wrong answer.

This tool takes density altitude as pressure altitude plus 120 feet per degree of ISA deviation, which is the rule the DGCA answer keys are built on. An ASA CX-3 device uses the exact ICAO relation instead and will differ from this by a couple of hundred feet at height. Both are defensible, and the exam wants the one above.

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