Density of Dry Air at 20 °C
| Value | 1.204 kg/m³ |
| Status | Measured: ± 0.002 kg/m³ (0.0017 relative) |
| Source | ASHRAE Handbook of Fundamentals, Ch. 1 |
| Categories | Material PropertiesEngineering & Trade |
| microgram per liter | 1,204,000 μg/L |
| milligram per cubic meter | 1,204,000 mg/m³ |
| milligram per liter | 1,204 mg/L |
| pound per cubic yard | 2.0294081 lb/yd³ |
| kilogram per cubic meter | 1.204 kg/m³ |
| gram per liter | 1.204 g/L |
| pound per cubic foot | 0.075163265 lb/ft³ |
| pound per imperial gallon | 0.012066985 lb/imp gal |
| pound per US gallon | 0.010047867 lb/gal |
| slug per cubic foot | 0.0023361457 slug/ft³ |
| gram per cubic centimeter | 0.001204 g/cm³ |
| gram per milliliter | 0.001204 g/mL |
| kilogram per liter | 0.001204 kg/L |
| tonne per cubic meter | 0.001204 t/m³ |
| US short ton per cubic yard | 0.0010147041 ton/yd³ |
| pound per cubic inch | 0.00004349726 lb/in³ |
Dry air at any temperature
-50 to 300 °CThe value above is one point on this curve. Dry air at 1 atm from −50 to 300 °C — the ideal-gas density and Sutherland viscosity behind every duct, fan and drag calculation.
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.