To begin to understand the calculation of air density,  consider the ideal gas law:

(1)      P*V = n*R*T

where:  P = pressure
V = volume
n = number of moles
R = gas constant
T = temperature

Density is simply the number of molecules of the ideal gas in a certain volume, in this case a molar volume, which may be mathematically expressed as:

(2)      D = n / V

where:  D = density
n = number of molecules
V = volume

Then, by combining the previous two equations, the expression for the density becomes:

(3)      

where:   D = density, kg/m³
P = pressure, Pascals ( multiply mb by 100 to get Pascals)
R = gas constant , J/(kg*degK) = 287.05 for dry air
T = temperature, deg K = deg C + 273.15

So lets translate this to an understandable meaning of Density Altitude.

In simplest terms, density altitude is the pressure altitude at a particular location that has been adjusted for current temperatures. It provides a more accurate description of the air pressure rather than simply how high something is above sea level. Temperature for the most part (and humidity to a lesser degree) is what causes changes in density altitude. If a vehicle is driving at an altitude of 5,000 feet and it is a cool 40 degrees F outside, the vehicle will perform like it is in fact at 5,000 feet. If however that same vehicle is driving at 5,000 feet and the temperature was closer to 100 degrees F on a hot summer day, the vehicle will instead perform like it is closer to 9,000 feet. The reason is because with higher temperatures, the particles in the air will be spaced further apart than if the air was colder. This results in less air being forced into the engine, and therefore less horsepower.

One of the most critical variables that can affect the performance of vehicles and humans is how much oxygen can be used by a vehicles engines or a persons lungs. Density altitude is a variable condition that can affect the amount of usable air at a particular altitude and temperature. In vehicles, density altitude can affect the power output of its engines. This change in performance can have a drastic impact on a vehicles acceleration speed and overall horsepower.

Since there is currently no instrumentation available for directly measuring density altitude, density altitude must be calculated. People generally rely on charts and tables to compare pressure altitude to density altitude, but density altitude can also be calculated with several formulas. The simplest of the formulas gives approximate density altitude as:

Density Altitude ft= Pressure Altitude ft + (120 X [actual temperature C – standard temperature C] ) ft

In order to find standard temperature, understand that standard temperature at sea level is 15 degrees C, and decreases an additional 2 degrees every 1000 feet.
So if someone is at an altitude of 6,000 feet and the current temperature was 35 C, the approximate density altitude would be:

6000 ft + (120 X [35 – (15-(2x6))])ft
6000 ft + (120 X [35 – 3])ft
6000 ft + (120 X 32)ft
6000 ft +3840 ft
9840 ft.