What kind of altitude is used to calculate density altitude?

Density altitude is calculated using pressure altitude. Pressure altitude is the height above the standard datum plane, which is a theoretical level where atmospheric pressure is 29.92 inches of mercury (Hg) or 1013.25 hPa. This metric is derived from the altimeter setting and is crucial for determining air density, which affects aircraft performance.

To calculate density altitude, one starts with pressure altitude and then adjusts for non-standard temperature using the International Standard Atmosphere (ISA) model. If the actual air temperature is warmer than standard temperature, the density altitude will be higher than the pressure altitude, resulting in reduced aircraft performance. Conversely, colder temperatures lead to a lower density altitude.

Understanding this concept is essential for pilots because density altitude significantly influences engine performance, climb rates, and overall aircraft handling characteristics, especially in hot and/or high-altitude conditions. Other altitude references, such as true altitude, absolute altitude, and indicated altitude, do not contribute to the calculations of density altitude as directly as pressure altitude does, making pressure altitude the correct parameter for this purpose.