The density ratio is also known as the air density ratio. The density ratio describes how the density of air at one location compares to that at another. In order to calculate air density, you need to know the temperature and the air pressure. The formula for air density is pressure divided by specific gas constant times the temperature. Air density looks at the mass per volume contained in the Earth's atmosphere in a given location.

Calculate the air density for location 1. Multiply the specific gas constant of dry air by the temperature. Then divide the pressure by the result. It is easier, though, to look at charts. The charts have temperature on one axis and pressure on another axis. This method will decrease any confusion between calculating between different units. For example, assume you have 13.6 Kilogram per square inch (psi) at 15.6 degrees Celsius. According to the Engineering Tool Box, the density of air in lb/ft^3 is 0.232.

Calculate the air density for location 2. Multiply the specific gas constant of dry air by the temperature. Then divide the pressure by the result. Again, it is easier to look at charts. For example, assume you have 60 psi at -1.11 degrees Celsius. According to the Engineering Tool Box, the density of air in lb/ft^3 is 0.412.

Divide the air density at location 2 by the air density at location 1. In the example, 0.412 divided by 0.232 equals a density ratio of 1.7758.

#### Warning

The references all use different units of measure. Be sure to check which units you are using.