Density altitude calculator
Enter the field elevation, the altimeter setting and the outside air temperature. You get pressure altitude and density altitude, with the rule of thumb beside the exact answer.
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In the app, pressure and density altitude are part of Headwind Pro, with true airspeed and Mach next to them. Headwind on Google Play.
For planning and training only, not for navigation. Check every result against your aircraft's flight manual and official sources before you rely on it.
How it is calculated
Pressure altitude comes first. It corrects the field elevation for today's pressure. Density altitude then corrects pressure altitude for temperature. This calculator uses the standard-atmosphere formulas:
station pressure = altimeter × (1 − 0.0000068756 × elevation)^5.2559 pressure altitude = 145442 × (1 − (station pressure ÷ 29.921)^0.19026) density altitude = 145442 × (1 − (17.326 × P ÷ (459.67 + T))^0.235)
P is the pressure in inHg at the pressure altitude. T is the temperature in °F. Results are in feet.
The rule of thumb you meet in training is shorter, and close enough to catch a problem:
pressure altitude ≈ elevation + (29.92 − altimeter) × 1000 standard temp (°C) ≈ 15 − 2 × (pressure altitude ÷ 1000) density altitude ≈ pressure altitude + 120 × (OAT − standard temp)
Worked example
A field at 5,355 ft, altimeter 30.12 inHg, 31 °C.
- Pressure altitude: the exact formula gives 5,178 ft. The rule of thumb gives 5,355 + (29.92 − 30.12) × 1000 = 5,155 ft.
- Standard temperature at that altitude is about 4.7 °C, so the air is 26.3 °C warmer than standard.
- Density altitude: the exact formula gives 8,140 ft. The rule of thumb gives 5,178 + 120 × 26.3, about 8,330 ft.
The aircraft is sitting at 5,355 ft but performs as if it were near 8,100 ft. The guide on what density altitude means for your takeoff explains why that matters.
Limits
- Dry air. Humidity is not included, so on a humid day the real figure is a little higher.
- It gives an altitude, not a takeoff distance. Takeoff and climb numbers come from your flight manual.
- Temperature near the runway surface can run warmer than the reported figure. Add a margin.
- At exactly standard conditions the formula lands within a few dozen feet of the pressure altitude rather than on it, because of rounding in the published constants. The same small difference shows in the app.
Frequently asked
What is density altitude, in one line? It is the altitude in the standard atmosphere at which the air would be as thin as the air around you now. It is what the wing, propeller and engine actually feel.
Why does this differ from the rule of thumb? The rule of thumb (120 ft per °C above standard) is a straight-line shortcut. The exact formula is not a straight line, so the two drift apart on hot days. In the example above they are about 190 ft apart. Both are shown so you can compare.
Does humidity count? Not here. This is the dry-air figure, the same one the rule of thumb gives. Humid air is a little thinner still, so on a muggy day the true figure is somewhat higher.
What altimeter setting do I enter? The current local setting from the METAR, ATIS or AWOS. If you have the altimeter in the aircraft set to 29.92 inHg (1013 hPa), the altitude it reads is the pressure altitude already.
Can I use density altitude to read my takeoff chart? Read how your chart is set up. Many light-aircraft performance tables are entered with pressure altitude and temperature, not density altitude. Use the flight manual's own method.