The density of air or atmospheric density, denoted ρ, is the mass per unit volume of Earth's atmosphere. Air density, like air pressure, decreases with increasing altitude. It also changes with variations in atmospheric pressure, temperature and humidity. At 101.325 kPa (abs) and 20 °C (68 °F), air has a density of approximately 1.204 kg/m³ (0.0752 lb/cu ft), according to the International Standard Atmosphere (ISA). At 101.325 kPa (abs) and 15 °C (59 °F), air has a density of approximately 1.225 kg/m³ (0.0765 lb/cu ft), which is about 1⁄800 that of water, according to the International Standard Atmosphere (ISA).^{[citation needed]} Pure liquid water is 1,000 kg/m³ (62 lb/cu ft).

Air density is a property used in many branches of science, engineering, and industry, including aeronautics;^[1][2][3] gravimetric analysis;^[4] the air-conditioning industry;^[5] atmospheric research and meteorology;^[6][7][8] agricultural engineering (modeling and tracking of Soil-Vegetation-Atmosphere-Transfer (SVAT) models);^[9][10][11] and the engineering community that deals with compressed air.^[12]

Depending on the measuring instruments used, different sets of equations for the calculation of the density of air can be applied. Air is a mixture of gases and the calculations always simplify, to a greater or lesser extent, the properties of the mixture.

Other things being equal, hotter air is less dense than cooler air and will thus rise through cooler air. This can be seen by using the ideal gas law as an approximation.

The density of dry air can be calculated using the ideal gas law, expressed as a function of temperature and pressure:^{[citation needed]}

$${\displaystyle {\begin{aligned}\rho &={\frac {p}{R_{\text{specific}}T}}\\R_{\text{specific}}&={\frac {R}{M}}={\frac {k_{\rm {B}}}{m}}\\\rho &={\frac {pM}{RT}}={\frac {pm}{k_{\rm {B}}T}}\\\end{aligned}}}$$

where:

• ${\displaystyle \rho }$, air density (kg/m³)^{[note 1]}
• ${\displaystyle p}$, absolute pressure (Pa)^{[note 1]}
• ${\displaystyle T}$, absolute temperature (K)^{[note 1]}
• ${\displaystyle R}$ is the gas constant, 8.31446261815324 in J⋅K⁻¹⋅mol^{−1 [note 1]}
• ${\displaystyle M}$ is the molar mass of dry air, approximately 0.0289652 in kg⋅mol⁻¹.^{[note 1]}
• ${\displaystyle k_{\rm {B}}}$ is the Boltzmann constant, 1.380649×10⁻²³ in J⋅K^{−1[note 1]}
• ${\displaystyle m}$ is the molecular mass of dry air, approximately 4.81×10⁻²⁶ in kg.^{[note 1]}
• ${\displaystyle R_{\text{specific}}}$, the specific gas constant for dry air, which using the values presented above would be approximately 287.0500676 in J⋅kg⁻¹⋅K⁻¹.^{[note 1]}

Therefore:

• At IUPAC standard temperature and pressure (0 °C and 100 kPa), dry air has a density of approximately 1.2754 kg/m³.
• At 20 °C and 101.325 kPa, dry air has a density of 1.2041 kg/m³.
• At 70 °F and 14.696 psi, dry air has a density of 0.074887 lb/ft³.

The following table illustrates the air density–temperature relationship at 1 atm or 101.325 kPa: ^{[citation needed]}

The addition of water vapor to air (making the air humid) reduces the density of the air, which may at first appear counter-intuitive. This occurs because the molar mass of water vapor (18 g/mol) is less than the molar mass of dry air^{[note 2]} (around 29 g/mol). For any ideal gas, at a given temperature and pressure, the number of molecules is constant for a particular volume (see Avogadro's Law). So when water molecules (water vapor) are added to a given volume of air, the dry air molecules must decrease by the same number, to keep the pressure or temperature from increasing. Hence the mass per unit volume of the gas (its density) decreases.

The density of humid air may be calculated by treating it as a mixture of ideal gases. In this case, the partial pressure of water vapor is known as the vapor pressure. Using this method, error in the density calculation is less than 0.2% in the range of −10 °C to 50 °C. The density of humid air is found by:^[13]

$${\displaystyle \rho _{\text{humid air}}={\frac {p_{\text{d}}}{R_{\text{d}}T}}+{\frac {p_{\text{v}}}{R_{\text{v}}T}}={\frac {p_{\text{d}}M_{\text{d}}+p_{\text{v}}M_{\text{v}}}{RT}}}$$

where:

• ${\displaystyle \rho _{\text{humid air}}}$, density of the humid air (kg/m³)
• ${\displaystyle p_{\text{d}}}$, partial pressure of dry air (Pa)
• ${\displaystyle R_{\text{d}}}$, specific gas constant for dry air, 287.058 J/(kg·K)
• ${\displaystyle T}$, temperature (K)
• ${\displaystyle p_{\text{v}}}$, pressure of water vapor (Pa)
• ${\displaystyle R_{\text{v}}}$, specific gas constant for water vapor, 461.495 J/(kg·K)
• ${\displaystyle M_{\text{d}}}$, molar mass of dry air, 0.0289652 kg/mol
• ${\displaystyle M_{\text{v}}}$, molar mass of water vapor, 0.018016 kg/mol
• ${\displaystyle R}$, universal gas constant, 8.31446 J/(K·mol)

The vapor pressure of water may be calculated from the saturation vapor pressure and relative humidity. It is found by:

$${\displaystyle p_{\text{v}}=\phi p_{\text{sat}}}$$

where:

• ${\displaystyle p_{\text{v}}}$, vapor pressure of water
• ${\displaystyle \phi }$, relative humidity (0.0–1.0)
• ${\displaystyle p_{\text{sat}}}$, saturation vapor pressure

The saturation vapor pressure of water at any given temperature is the vapor pressure when relative humidity is 100%. One formula is Tetens' equation from^[14] used to find the saturation vapor pressure is:

$${\displaystyle p_{\text{sat}}=6.1078\times 10^{\frac {7.5T}{T+237.3}}}$$

where:

• ${\displaystyle p_{\text{sat}}}$, saturation vapor pressure (hPa)
• ${\displaystyle T}$, temperature (°C)

See vapor pressure of water for other equations.

The partial pressure of dry air ${\displaystyle p_{\text{d}}}$ is found considering partial pressure, resulting in:

$${\displaystyle p_{\text{d}}=p-p_{\text{v}}}$$

where ${\displaystyle p}$ simply denotes the observed absolute pressure.

• Air
• Atmospheric drag
• Lighter than air
• Density
• Atmosphere of Earth
• International Standard Atmosphere
• U.S. Standard Atmosphere
• NRLMSISE-00