Density altitude

Density altitude is the altitude in the standard atmosphere at which the air would have the same density as the current air. It combines pressure altitude and temperature. A high density altitude means thinner air, even when the aircraft is physically at a much lower elevation.

How it works

Density altitude starts with pressure altitude, which removes the local altimeter setting from field elevation. Temperature then shifts the result. Warmer-than-standard air produces a higher density altitude. Colder-than-standard air produces a lower one.

Thin air reduces the mass of air entering a normally aspirated engine, reduces propeller thrust, and reduces wing lift for a given true airspeed. The aircraft must move through the air faster to produce the same lift, which increases takeoff distance and reduces climb performance. The actual result depends on the aircraft, loading, runway, wind, and condition of the engine and propeller.

Use the density altitude calculator for a transparent estimate. It uses field elevation, altimeter setting, and outside air temperature. The result is a planning value, not an aircraft performance guarantee.

Example

Assume a field elevation of 5,000 ft, an altimeter setting of 29.92 inHg, and an outside air temperature of 30°C. Because the pressure altitude is about 5,000 ft and the temperature is warmer than standard, the density altitude will be materially higher than field elevation. A pilot would compare the result with the aircraft POH performance charts for the actual weight and runway conditions.

Why it matters

Two airports at the same physical elevation can have different density altitudes when their temperatures or pressure settings differ. The difference can also become large during hot weather at an elevated airport.

Density altitude does not describe the aircraft's height above the ground. For that reference, see pressure altitude. Verify performance with the applicable POH or AFM and current conditions.

Related terms and tools

• Pressure altitude
• Indicated airspeed
• Density altitude calculator
• FAA handbooks and manuals