What is the density of air at STP? In order to answer the question, you need to understand what density is and how STP is defined.

### Key Takeaways: Density of Air at STP

- The value for the density of air at STP (Standard Temperature and Pressure) depends on the definition of STP. The definition of the temperature and pressure isn't standard, so the value depends on who you consult.
- The ISA or International Standard Atmosphere states the density of air is 1.225 kg/m3 at sea level and 15 degrees C.
- The IUPAC uses an air density of 1.2754 kg/m3 at 0 degrees C and 100 kPa for dry air.
- Density is affected not only by temperature and pressure but also by the amount of water vapor in the air. Thus, the standard values are only an approximation.
- The Ideal Gas Law may be used to calculate density. Once again, the result is only an approximation that is most accurate at low temperature and pressure values.

The density of air is the mass per unit volume of atmospheric gases. It is denoted by the Greek letter rho, ρ. The density of air, or how light it is, depends on the temperature and pressure of the air. Typically, the value given for the density of air is at STP (standard temperature and pressure).

STP is one atmosphere of pressure at 0 degrees C. Since this would be a freezing temperature at sea level, dry air is less dense than the cited value most of the time. However, air typically contains a lot of water vapor, which would make it denser than the cited value.

## The Density of Air Values

The density of dry air is 1.29 grams per liter (0.07967 pounds per cubic foot) at 32 degrees Fahrenheit (0 degrees Celsius) at average sea-level barometric pressure (29.92 inches of mercury or 760 millimeters).

- At sea level and at 15 degrees C, the density of air is 1.225 kg/m
^{3}. This is the value of the ISA (International Standard Atmosphere). In other units, this is 1225.0 g/m^{3}, 0.0023769 slug/(cu ft), or 0.0765 lb/(cu ft). - The IUPAC standard of temperature and pressure (0 degrees C and 100 kPa), uses a dry air density of 1.2754 kg/m
^{3}. - At 20 degrees C and 101.325 kPa, the density of dry air is 1.2041 kg/m
^{3}. - At 70 degrees F and 14.696 psi, the density of dry air is 0.074887 lbm/ft
^{3}.

## Affect of Altitude on Density

The density of air decreases as you gain altitude. For example, the air is less dense in Denver than in Miami. The density of air decreases as you increase temperature, providing the volume of the gas is allowed to change. As an example, air would be expected to be less dense on a hot summer day versus a cold winter day, providing other factors remain the same. Another example of this would be a hot air balloon rising into a cooler atmosphere.

## STP Versus NTP

While STP is standard temperature and pressure, not many measured processes occur when it's freezing. For ordinary temperatures, another common value is NTP, which stands for normal temperature and pressure. NTP is defined as air at 20 degrees C (293.15 K, 68 degrees F) and 1 atm (101.325 kN/m^{2}, 101.325 kPa) of pressure. The average density of air at NTP is 1.204 kg/m^{3}** **(0.075 pounds per cubic foot).

## Calculate the Density of Air

If you need to calculate the density of dry air, you can apply the ideal gas law. This law expresses density as a function of temperature and pressure. Like all gas laws, it is an approximation where real gases are concerned but is very good at low (ordinary) pressures and temperatures. Increasing temperature and pressure adds error to the calculation.

The equation is:

ρ = p / RT

where:

- ρ is the air density in kg/m
^{3} - p is the absolute pressure in Pa
- T is the absolute temperature in K
- R is the specific gas constant for dry air in J/(kg·K) or is 287.058 J/(kg·K).

## Sources

- Kidder, Frank E. "Kidder-Parker Architects' and Builders' Handbook, Data for Architects, Structural Engineers, Contractors and Draughtsmen." Harry Parker, Hardcover, Twelfth Printing of the 18th Edition edition, John Wiley & Sons, 1949.
- Lewis Sr., Richard J. "Hawley's Condensed Chemical Dictionary." 15th Edition, Wiley-Interscience, January 29, 2007.