Air Density Calculator
Density of moist air from temperature, pressure and relative humidity (or dew point) — with the dry-air value, water-vapour partial pressure, specific volume and altitude correction.
How it works
ρ = p_d ÷ (R_d·T) + p_v ÷ (R_v·T), R_d = 287.06, R_v = 461.5 J/(kg·K); p_v = RH × e_s(T) (Magnus); p_d = P − p_v
Air density falls as temperature rises and pressure drops, and — counter-intuitively — as humidity rises, because water molecules (18 g/mol) displace heavier nitrogen and oxygen (29 g/mol). The effect matters for aircraft performance (density altitude), engine tuning, wind-turbine output and ballistics. The calculation treats dry air and water vapour as separate ideal gases; the Magnus formula supplies the saturation vapour pressure.
Worked example
25 °C, 1013.25 hPa, 50% RH: e_s = 31.7 hPa so p_v = 15.8 hPa; ρ = 1.1685 + 0.0115 = 1.180 kg/m³, 0.5% lighter than dry air and 96% of standard.
Frequently asked questions
Should I use sea-level or station pressure?
Station (absolute) pressure. Weather reports usually give sea-level-corrected values — subtract about 12 hPa per 100 m of elevation.
How much does altitude change density?
Roughly 1.2% per 100 m near sea level; at 2,000 m the air is about 80% as dense.
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This calculator is for educational purposes. Double-check important results before acting on them.