Density of Dry Air at 20 °C

\rho_{\mathrm{air}} = 1.204\ \text{kg/m}^{3}

Value	1.204 kg/m³
Status	Measured: ± 0.002 kg/m³ (0.0017 relative)
Source	ASHRAE Handbook of Fundamentals, Ch. 1
Categories	Material Properties Engineering & Trade hvac

ρ_air in every density unit

kilogram per cubic meter	1.204 kg/m³
gram per cubic centimeter	0.001204 g/cm³
gram per milliliter	0.001204 g/mL
kilogram per liter	0.001204 kg/L
pound per cubic foot	0.075163265 lb/ft³
pound per US gallon	0.010047867 lb/gal

Learning zone

Air density follows the ideal gas law closely enough that you can compute it rather than look it up: ρ = p/(R T) with R = 287.05 J/(kg·K) for dry air. At 20 °C and one atmosphere that gives 1.204 kg/m³, or 0.0752 lb/ft³ — the "standard air" of North American HVAC, usually quoted as 0.075 lb/ft³ at 70 °F. Every familiar duct constant descends from it: 1.08 for sensible heat, 4.5 for total heat, 0.68 for latent.

The trap is altitude and temperature. Air at 1500 m is about 16 % less dense than at sea level, and a furnace flue at 200 °C is 40 % of standard density. A fan is a constant-volume machine, so it still moves the same cfm — but it moves proportionally less mass, delivers proportionally less heat, and draws proportionally less brake horsepower. Denver-based CFM ratings and sea-level heat calculations are the classic mismatch. Humidity moves it the other way, slightly: moist air is lighter than dry air at the same temperature and pressure because water vapour is lighter than the nitrogen and oxygen it displaces.

Used by these solvers

Air Sensible Heat (1.08 Rule)

\dot{Q}_s = \rho_a c_a \dot{V} \, \Delta T

Density

\rho = \tfrac{m}{V}

Drag Force (F = ½CdρAv²)

F_D = \tfrac{1}{2} C_d \rho A v^{2}

Dynamic Pressure (q = ½ρv²)

q = \tfrac{1}{2} \rho v^{2}

Reynolds Number

Re = \frac{\rho v D}{\mu}