Density Altitude Calculator

The density altitude formula

Density altitude is found by comparing the actual air temperature with the standard (ISA) temperature at the same pressure altitude. First compute the ISA temperature, then add an altitude correction for every degree the air is warmer than standard:

ISA temp (°C) = 15 − 1.98 × (PA / 1000)
DA (ft)       = PA + 120 × (OAT − ISA temp)

Where PA is the pressure altitude in feet, OAT is the outside air temperature in °C, and DA is the density altitude in feet. The standard atmosphere assumes a sea-level temperature of 15 °C and a lapse rate of about 1.98 °C per 1,000 ft. Each degree Celsius above standard raises the density altitude by roughly 120 ft. This is the same rule-of-thumb pilots use in the cockpit, and it is accurate enough for performance planning across normal operating temperatures.

Worked example

Take a field with PA = 5,000 ft on a hot afternoon at OAT = 30 °C:

• ISA temp = 15 − 1.98 × (5000 / 1000) = 15 − 9.9 = 5.1 °C
• DA = 5000 + 120 × (30 − 5.1) = 5000 + 120 × 24.9 = 5000 + 2,988 ≈ 7,988 ft

So at a 5,000 ft pressure altitude and 30 °C, the aircraft performs as if it were at nearly 8,000 ft, a serious hit to climb rate and takeoff distance.

Density altitude reference (PA = 5,000 ft)

Pitfalls to avoid

• Use pressure altitude, not indicated altitude. Set the altimeter to 29.92 inHg first, or the result will be off by the local pressure error.
• Humidity is ignored. Moist air is slightly less dense, so on very humid days the true density altitude is a touch higher than this standard formula shows.
• Density altitude can be negative. On a cold day the air is denser than standard, giving a density altitude below the pressure altitude, that is normal and means better performance.
• Always cross-check the POH. This estimate is for planning; your aircraft flight manual charts are the authority for takeoff and climb numbers.