Density altitude calculator
Pick an airport and see the density altitude and pressure altitude from the current METAR.
This ignores humidity. So does your POH. On a humid day the real density altitude is higher than either one says.
The formula
pressure altitude = elevation + 145,366.45 × (1 − (setting / 1013.25)0.190284)
density altitude = pressure altitude + 118.8 × (temperature − standard)
standard temperature = 15 − 1.98 × pressure altitude / 1000
The altimeter setting is in hectopascals (1013.25 is standard sea level), and the temperature in degrees C. Standard temperature drops by about 2 °C per thousand feet. Every degree above standard adds 118.8 ft of density altitude.
At Centennial (5,885 ft) with 29.92 inHg and 31 °C: pressure altitude is 5,885 ft (standard pressure, so no correction), standard temperature at that altitude is 3.3 °C, the air is 27.7 °C above standard, and the density altitude works out to about 9,180 ft. That is a 56% longer ground roll than the field elevation alone suggests.
How this works
- What density altitude is
- Density altitude is the altitude at which the air density matches standard conditions. On a hot day at a high field, the air is thinner than the elevation suggests. The engine makes less power, the wings make less lift, and the propeller moves less air. The aircraft performs as if it were at a higher airport.
- How the calculation works
- Start with pressure altitude. Subtract the altimeter setting from the standard 29.92 inHg and multiply by 1,000. Add the field elevation. Then correct for temperature: find how far the actual temperature is from standard at that pressure altitude, and apply the ISA deviation. This is the same method the performance charts in the POH use.
- Humidity is not included
- Water vapour is lighter than dry air. On a humid day the air is less dense and the real density altitude is higher than the standard formula says, by up to a few hundred feet. The standard calculation does not account for it. Neither do POH performance charts. The error works against you, on the hot summer days when the margin is already thin.
The aircraft performs as if the airport were at 9,180 ft.
Not a substitute for an official pre-flight weather briefing.