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Air Density Calculator

Computes the density of humid air from temperature, station pressure and relative humidity as a mixture of ideal gases (dry air plus water vapour, vapour pressure from the Magnus formula), and the ISA density altitude.

When to use

You need air density for aerodynamics, engine or fan performance, ballistics or HVAC, or the density altitude for aircraft performance planning.

Do not use when: You only have sea-level (QNH) pressure and no elevation (convert to station pressure with pressure-altitude first), or you need gas density for a gas other than air (use ideal-gas-law).

Formula

pv = RH/100 · 6.1094·exp(17.625·T/(243.04 + T)) hPa; ρ = (p − pv)·100/(287.058·T_K) + pv·100/(461.495·T_K); density altitude = 44330.8 × [1 − (ρ/1.225)^0.23497] m

Ideal-gas mixture with Rd = 287.058 and Rv = 461.495 J/(kg·K); the compressibility factor (≈ 0.9996 at sea level) is ignored. Density altitude inverts the ISA troposphere density profile (exponent 1/(g·M/(R·L) − 1)).

Inputs

ParameterTypeUnitRequiredDescription
temperaturenumberyesAir temperature in temperature_unit. Range: ≥ -80, ≤ 140
temperature_unitenum: c | fdefault cUnit of the temperature input(s).
pressurenumberyesActual barometric pressure at the location, in pressure_unit (not the sea-level-corrected QNH/altimeter setting). Range: > 0, ≤ 1200
pressure_unitenum: hpa | kpa | inhg | mmhgdefault hpaUnit of the pressure input.
relative_humidity_percentnumber%default 0Relative humidity in percent; 0 gives dry air. Range: ≥ 0, ≤ 100

Outputs

OutputTypeUnitDescription
air_density_kg_m3numberkg/m³Density of the moist air.
air_density_lb_ft3numberlb/ft³Same density in pounds per cubic foot.
dry_air_density_kg_m3numberkg/m³Density at the same temperature and pressure with 0 % humidity, for comparison.
vapor_pressure_hpanumberhPaPartial pressure of water vapour.
percent_of_isa_sea_levelnumber%Density as a percentage of 1.225 kg/m³ (ISA, 15 °C, 1013.25 hPa, dry).
density_altitude_mnumbermAltitude in the ISA at which dry air would have this density.
density_altitude_ftnumberftDensity altitude in feet.

Example

ISA sea level: 15 °C, 1013.25 hPa, dry: {"temperature":15,"temperature_unit":"c","pressure":1013.25,"pressure_unit":"hpa","relative_humidity_percent":0} → {"air_density_kg_m3":1.225,"dry_air_density_kg_m3":1.225,"percent_of_isa_sea_level":100,"density_altitude_m":0,"density_altitude_ft":0}

30 °C, 1000 hPa, 50 % RH: {"temperature":30,"temperature_unit":"c","pressure":1000,"pressure_unit":"hpa","relative_humidity_percent":50} → {"air_density_kg_m3":1.1399,"dry_air_density_kg_m3":1.1491,"vapor_pressure_hpa":21.18,"air_density_lb_ft3":0.07116,"density_altitude_m":743,"density_altitude_ft":2439}

GET https://tttkmbb.com/api/v1/calculate/air-density?temperature=15&temperature_unit=c&pressure=1013.25&pressure_unit=hpa&relative_humidity_percent=0

Machine access

Sources

FAQ

Why is humid air lighter than dry air?

Water vapour (molar mass 18 g/mol) displaces nitrogen and oxygen (29 g/mol average) at the same pressure, so the mixture has lower density; at 30 °C the effect is about 1 %.

Which pressure should I enter?

The station (absolute) pressure. Weather reports usually give sea-level-reduced pressure; use pressure-altitude with your elevation to derive the station pressure first.

How does density altitude affect aircraft?

Higher density altitude means less lift, less engine power and less propeller thrust: takeoff distances grow and climb rates fall; pilots use it in performance charts.

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