What density altitude tells you
Airplanes perform according to air density, not the altitude on the altimeter. Hot air, high elevation and low pressure all thin the air. The engine makes less power, the propeller less thrust and the wing less lift at a given airspeed, so takeoff rolls get longer and climbs get shallower. Density altitude sums all three effects into one number: the standard-atmosphere altitude with the same air density.
How it is calculated
- Pressure altitude. The field elevation is corrected for the altimeter setting. An altimeter set to 30.10 inHg reads 165 feet lower than one set to 29.92, so a 5,883-foot field is at a pressure altitude of 5,718 feet. That matches the FAA's altimeter-correction table.
- Temperature deviation. The standard temperature at 5,718 feet is 3.7 °C. At 21.1 °C (70 °F) the air is 17.4 °C warmer than standard.
- Density. The actual air density is pressure divided by the gas constant times the absolute temperature. Here it is 79.3% of standard sea-level density.
- Density altitude. The calculator finds the standard-atmosphere altitude with that same density: 7,708 feet. The PHAK's density altitude chart gives about 7,700 feet for the same problem (Sample Problem 1).
With a dewpoint entered, the calculator uses the virtual temperature: the temperature dry air would need to have the same density as the moist air. That is how the National Weather Service handles humidity, and the result lands within 30 feet of the NWS formula's value in the PHAK's example.
The rule of thumb and the E6B
The classroom shortcut adds 120 feet for every degree Celsius above standard: 5,718 + 120 × 17.4 = 7,810 feet. It is quick and conservative in hot weather, and the result shows it for comparison.
On a mechanical E6B the same problem uses the altitude window: set pressure altitude over temperature and read density altitude against its index. The E6B simulator animates exactly that, and its reading matches this calculator's dry value.
Where density altitude bites
High-elevation airports on summer afternoons are the classic trap: at a pressure altitude of 5,000 feet and 30 °C, the density altitude is about 7,800 feet. The same airplane that climbed well at sea level may barely climb. Plan for it with your POH's performance charts, reduce weight, and depart early in the day when it is cooler.
Density altitude on a mechanical E6B
- Find the pressure altitude: set 29.92 on the altimeter and read it, or apply the altimeter correction to the field elevation.
- In the altitude window on the calculator side, turn the disc until the pressure altitude sits over the outside air temperature.
- Read density altitude in the density-altitude window against its index.
Questions pilots ask
What is density altitude?
Density altitude is the altitude in the standard atmosphere where the air has the same density as the air around you. An airplane's wings, propeller and engine perform as if it were at that altitude.
Does humidity really matter?
Somewhat. Water vapor is lighter than dry air, so humid air is less dense. In the PHAK's example at 8,000 feet, 80 °F and a 75 °F dewpoint, humidity adds almost 500 feet of density altitude. Enter the dewpoint to include it.
Why is the 120 feet per degree rule different from the result?
The rule of thumb is a linear approximation that most E6B problems and knowledge-test questions accept. The exact calculation inverts the standard atmosphere, so the two drift apart as the temperature departs from standard. In the example the rule gives 7,810 feet and the exact dry value is 7,708 feet.
What density altitude is too high?
There is no single number: it depends on the airplane, its weight and the runway. Use the density altitude with your POH's takeoff and climb charts, which are built on pressure altitude and temperature for this reason.