Density Altitude - Calculation

Calculation

Density altitude can be calculated from atmospheric pressure and temperature (assuming dry air).

where

density altitude in feet

atmospheric (static) pressure

standard sea level atmospheric pressure (1013.25 hPa ISA or 29.92126 inHg US))

true (static) air temperature in kelvins (K) figure

ISA standard sea level air temperature in kelvins (K) (288.15 K)

0.234969

Note that the leading coefficient is the ratio of the ISA sea level temperature (288.15 K) to ISA Temperature Lapse Rate (6.5 K/km), converted into feet.

National Weather Service Equation

The National Weather Service uses the following dry-air approximation of the above equation in their standards.

where

density altitude in feet

Is the station pressure (atmospheric static pressure) in inches of mercury (inHg)

T is the station temperature (atmospheric temperature) in Fahrenheit (F)

Note that the NWS standard specifies that the density altitude should be rounded to the nearest 100 feet.

Easy formula to calculate density altitude from pressure altitude

This is an easier formula to calculate (with great approximation) density altitude from pressure altitude ..and International Standard Atmosphere temperature deviation

Density altitude in feet = pressure altitude in feet + 118.8 x (OAT - ISA_temperature)

Where:

OAT = Outside air temperature in °C

ISA_temperature = 15 °C - 1.98ºC / 1000ft x PA

considering that temperature drops at the rate of 1.98 °C each 1000 ft of altitude until the Tropopause (36000ft), usually rounded to 2ºC

Or simply:

DA=PA+118.8(+OAT-15)

Or even simpler

DA=1.24 PA + 118.8 OAT - 1782

where DA=density altitude and PA=pressure altitude where PA=Hgt+30(1013-QNH)