How to use the density altitude calculator
- Enter the field elevation and the altimeter setting, choosing the unit for each.
- Enter the outside air temperature in Celsius or Fahrenheit. Open the advanced option to add a dew point for the air-density figure.
- Read the density altitude from the common rule of thumb, with the pressure altitude and the temperature difference from standard.
- Compare it with the density altitude worked out from the air density itself. The page shows how far apart the two methods are.
Formula
pressure altitude = elevation + (29.92 - altimeter in inHg) x 1,000 ft
standard temperature = 15 - 1.98 x (pressure altitude / 1,000) in Celsius
density altitude = pressure altitude + 120 x (air temperature - standard temperature)
cross-check: air density = pressure / (R x T), then the altitude in a standard atmosphere with that density
The first three lines are the common rules of thumb. The cross-check uses the gas law, a vapor-pressure term when you give a dew point, and the standard atmosphere formulas.
A field at 5,000 feet on a 30 degree day
Take a field elevation of 5,000 ft, an altimeter setting of 29.92 inHg and an air temperature of 30 C. Pressure altitude is 5,000 ft. Standard temperature there is 15 - 1.98 x 5 = 5.1 C, so the air is 24.9 C warmer than standard, and density altitude is 5,000 + 120 x 24.9 = 7,988 ft.
The gas-law cross-check gives a station pressure of 843 hPa, an air density of 0.9688 kg/m3, which is 79 percent of sea-level standard, and a density altitude near 7,800 ft. A dew point of 20 C raises that to about 8,140 ft. At sea level with 30.12 inHg and 15 C, the rule gives -248 ft.
Why density altitude matters
Thin air has less to work with: an engine makes less power, a propeller or wing gets less thrust or lift, and takeoff and climb performance fall. High density altitude, which comes from high elevation, low pressure and high heat, is a known problem for small aircraft on hot days at high fields. The same idea helps with other air-dependent machines such as engines and fans, and with the thinner air at altitude. See also the boiling point at altitude calculator and the dew point calculator.
Learning aid only
This page is a learning aid and a rough estimate. It is not for flight planning. Use the aircraft handbook, current weather and your own judgment, and ask an instructor. The 120 feet per degree rule is an approximation, which is why the page also shows the figure from air density. For other air and navigation ideas, see the wind triangle calculator and the flash guide number calculator.
Assumptions and limits
- Elevation must be between -1,500 and 20,000 feet, and the pressure altitude must stay within the range where the rules of thumb apply.
- Standard temperature falls 1.98 C per 1,000 ft, which holds in the lower atmosphere only.
- The altimeter setting is converted at 29.92 inHg = 1013.25 hPa. The station pressure assumes a standard atmosphere between the field and sea level.
- A dew point is optional, must not exceed the air temperature, and affects only the air-density figure.
Frequently asked questions
Why are there two density altitude figures?
The rule of thumb is simple and widely taught. The other figure comes from the gas law and the standard atmosphere, so it is closer to the physics. The difference shows how rough the rule is.
Does humidity matter?
Humid air is slightly less dense than dry air at the same temperature and pressure, so humidity raises density altitude a little. Enter a dew point to see the effect on the air-density figure.
Can I use meters and hectopascals?
Yes. Choose meters for the elevation and hectopascals for the pressure, and Fahrenheit if you prefer. The page converts and shows results in your elevation unit.
Is this safe to use for flying?
No. It is a learning aid. Always use approved performance data for your aircraft and the weather for your field.