Free Air Density calculator with clear step-by-step results.
Air density comes from the ideal gas law, not from weighing a sample: ρ = p ÷ (R × T) with absolute temperature in kelvin. Humidity lowers it, which is counter-intuitive — water vapour has a lower molar mass than the nitrogen and oxygen it displaces, so humid air is lighter than dry air at the same pressure and temperature. The calculation therefore splits the total pressure into dry and vapour partial pressures and applies each gas constant separately.
Saturation and vapour pressure
Psat = 6.1078 × 10^(7.5T ÷ (T + 237.3)); Pv = Psat × RH ÷ 100
Moist air density
ρ = Pd ÷ (287.058 × T) + Pv ÷ (461.495 × T), pressures in Pa and T in kelvin
Uses the Tetens approximation for saturation vapour pressure and treats air as an ideal gas. For metrology, aviation performance or certification work use the full CIPM formulation and calibrated instruments.
Water vapour has a molar mass of 18 g/mol against about 29 for dry air. At a given pressure and temperature the total molecule count is fixed, so replacing heavy air molecules with lighter water molecules reduces the mass per cubic metre.
1.225 kg/m³ at 15 °C and 1013.25 hPa, the ISA sea-level condition. The final result shows your conditions as a percentage of that, which is the quantity that drives lift, drag and engine output.
Station pressure — the actual absolute pressure where you are. Sea-level-corrected pressure from a weather report will overstate density at altitude, sometimes substantially.