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Dry air at 15 °C (59 °F) and 1 atm has a density of 1.2255 kg/m³ (0.07651 lb/ft³). At 20 °C it is 1.2046 kg/m³. Density falls as air gets hotter and rises with pressure.
This page is for dry air. Humid air is lighter. For humid air use the wet bulb calculator. For density at altitude use the density altitude calculator.
For an ideal gas, density is set by the ideal gas law:
ρ = P / (R·T)
with P in Pa, T in K and R = 287.05 J/(kg·K) for dry air (8.31432 J/(mol·K) divided by 28.9644 g/mol, from the U.S. Standard Atmosphere 1976). Real air is described with the compressibility factor Z:
ρ = P / (Z·R·T)
At 1 atm the ideal gas law is within about 0.1 % of the real value above 0 °C. At 100 bar and −100 °C, Z is 0.659, so the ideal gas law is about 52 % too high there. At 100 bar and 1000 °C, Z is 1.027. Use the real-gas value above about 10 bar or below about −50 °C.
Accuracy. Lemmon et al. (2000) state an uncertainty of about 0.1 % in density, 0.2 % in speed of sound and 1 % in heat capacity from the solidification point to 873 K (600 °C) and up to 70 MPa. Above 873 K there were no reliable air data, so nitrogen data were used and the paper gives about 0.5 % for density. Lemmon & Jacobsen (2004) state about 5 % for the viscosity and thermal conductivity of air, and more near the critical point. Viscosity and conductivity above about 600 °C, and the 1000 °C end of the range, are less certain than that. CoolProp uses a molar mass of 28.96546 g/mol, the U.S. Standard Atmosphere uses 28.9644 g/mol. That is the main reason CoolProp density at 15 °C and 1 atm is 0.04 % above the Standard Atmosphere value.