Density altitude calculator
Density altitude is the altitude your aircraft performs as if it were at. It folds temperature into pressure altitude, which is why a hot day at a modest airfield can feel like taking off thousands of feet higher.
Calculator
This is an educational and planning aid. Use current official weather, your aircraft flight manual, and the rules that apply to your flight before you act on any figure here. See the aviation disclaimer.
What the result means
The answer is the altitude in the standard atmosphere where the air has the same density as the air you are sitting in. If your field is at 5,000 ft and the density altitude comes out at 8,500 ft, your engine, propeller and wing all behave as though you were at 8,500 ft.
Three things get worse together: the engine makes less power, the propeller bites less air, and the wing needs a higher true airspeed to produce the same lift. Takeoff roll lengthens, climb rate falls, and the margin over obstacles shrinks.
When this is useful
- On any warm day at a high airfield, before committing to a takeoff with a full load.
- Whenever a performance chart asks for density altitude directly rather than pressure altitude and temperature separately.
- When comparing how an aircraft felt on two different days, since density altitude explains most of the difference.
How it is calculated
Pressure altitude comes first, from field elevation and the altimeter setting. That gives the static pressure of the air you are in, using the standard atmosphere.
That pressure and your measured temperature give the actual air density, through the ideal gas law. The calculator then finds the altitude in the standard atmosphere at which that density occurs, and that altitude is the answer.
This is the full method used by meteorological services. The familiar shortcut of adding 120 ft for every degree above standard is a reasonable approximation near sea level, but it drifts by a few hundred feet in hot, high conditions.
density = pressure / (287.05 x temperature in kelvin), then read that density back off the standard atmosphere
Worked example
- Field elevation
- 5,431 ft
- Altimeter setting
- 1013.25 hPa
- Outside air temperature
- 32.2 C
Density altitude is about 8,555 ft, roughly 3,100 ft above the field elevation. The aircraft will perform as though it were at 8,555 ft on a standard day.
Check: This is the classic hot day at a mile high airfield, widely quoted as giving a density altitude near 8,500 ft. The rule of thumb gives 5,431 plus 120 multiplied by 28, which is about 8,800 ft, a few hundred feet high as expected.
Limitations
- The calculation assumes dry air. Humid air is less dense than dry air at the same temperature and pressure, so on a muggy day the true density altitude is a few hundred feet higher than shown.
- It tells you what the air is doing, not what your aircraft will do in it. Only your aircraft flight manual can turn density altitude into a takeoff distance or a climb rate.
- Temperature must be the actual temperature at the airfield. A forecast figure, or a reading from a sun soaked apron thermometer, will both mislead.
Common mistakes
- Using the temperature in Fahrenheit while the field is set to Celsius. Use the unit selector rather than converting by hand.
- Assuming a high density altitude only matters for takeoff. It also lengthens the landing roll and flattens the go around climb, which is when the margin is thinnest.
- Treating the result as a limit. It is an input to a performance chart, not a verdict on whether the flight is safe.
Sources
- Pilot's Handbook of Aeronautical Knowledge, chapter 4 , US Federal Aviation Administration
- Density Altitude (Safety Brochure P-8740-02) , US Federal Aviation Administration
- Earth Atmosphere Model , NASA Glenn Research Center