What Is the Density of Air at STP?

Question: What Is the Density of Air at STP?

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

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

• At sea level and at 15°C , the density of air is 1.275 kg/m³. This is the value of the ISA or International Standard Atmosphere.
• At 20 °C and 101.325 kPa, the density of dry air is 1.2041 kg/m³.
• At 70 °F and 14.696 psi, the density of dry air is 0.074887 lbm/ft³.

The density of air decreases as you gain altitude. For example, 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.

References:
Kidder, Frank. Architects' and Builders' Handbook, p. 1569.
Lewis, Richard J., Sr. Hawley's Condensed Chemical Dictionary, 12th ed., p. 28.

By Anne Marie Helmenstine, Ph.D.

Chemistry Expert