Does humidity increase or decrease air density?

Humid air is less dense than dry air at the same temperature and pressure. Water vapour (molar mass 18 g/mol) is lighter than the average dry air mixture (effective molar mass about 28.97 g/mol). When water vapour displaces heavier nitrogen and oxygen molecules, the overall density of the air parcel decreases. This is why aircraft performance is lower on hot, humid days: the engines and wings are working in less dense air.

Why does air density decrease with altitude?

Atmospheric pressure decreases with altitude because there is less weight of air above. The ideal gas law (density = pressure divided by (specific gas constant multiplied by temperature)) shows that as pressure drops, density falls proportionally (assuming the same temperature). In practice, temperature also decreases with altitude in the troposphere (about 6.5 °C per 1,000 m), which partially counteracts the pressure effect but the pressure drop is dominant, so density still falls significantly with height.

Air Density Calculator

Calculate the density of air (kg/m³) for any combination of temperature, pressure, and relative humidity. Uses the ideal gas law for dry air and the Buck equation for moist air partial pressures.

At standard sea-level conditions (15 °C, 101,325 Pa, 0% humidity), dry air has a density of 1.225 kg/m³.

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Reviewed June 2026 Standard atmosphere values from ISO 2533:1975 and ICAO Doc 7488/3.

1. Temperature and Pressure

Temperature

°C

Temperature unit

Pressure

Pressure unit

2. Humidity and Presets

Relative humidity (%)

Quick presets

Air Density Results

Air Density
1.225
kg/m³

Density (g/cm³)

0.001225

g/cm³

Temperature (K)

288.15

Kelvin

Pressure (hPa)

1013.25

hPa / mbar

Calculation Breakdown

Temperature (K)288.15

Pressure (Pa)101,325

Relative humidity0%

Saturation vapour pressure0.00 Pa

Partial pressure of water vapour0.00 Pa

Partial pressure of dry air101,325 Pa

Density (kg/m³)1.2250

Reference Values

ISA sea level (15 °C, 1 atm)1.2250 kg/m³

NIST STP (20 °C, 1 atm)1.2041 kg/m³

IUPAC STP (0 °C, 100 kPa)1.2754 kg/m³

Hot humid (30 °C, 80% RH)~1.1496 kg/m³

2,000 m altitude (ISA)~1.0066 kg/m³

Specific gas constant, dry air (R_d)287.058 J/(kg·K)

Specific gas constant, water (R_v)461.495 J/(kg·K)

Result: At 15 °C and 101,325 Pa with 0% relative humidity, air density is 1.2250 kg/m³, matching the International Standard Atmosphere sea-level value.

How Air Density is Calculated

Air density (symbol rho, Greek letter) is the mass of air per unit volume, measured in kilograms per cubic metre (kg/m³). It depends on three variables: temperature, pressure, and humidity. The standard approach uses the ideal gas law, which is accurate to better than 0.1% for typical atmospheric conditions.

Formula for Dry Air

For perfectly dry air (zero humidity), the density is:

rho = p / (R_d x T)

Where:

rho = air density (kg/m³)
p = absolute pressure (Pa)
R_d = specific gas constant for dry air = 287.058 J/(kg·K)
T = absolute temperature (Kelvin = °C + 273.15)

Formula for Moist Air

Humid air is a mixture of dry air and water vapour. Because water vapour (molar mass 18.015 g/mol) is lighter than the average dry air mixture (effective molar mass 28.97 g/mol), adding water vapour reduces density. The formula accounts for this using the partial pressures of dry air and water vapour separately:

rho = (p_d / (R_d x T)) + (p_v / (R_v x T))

Where:

p_d = partial pressure of dry air = total pressure minus vapour pressure (Pa)
p_v = partial pressure of water vapour = relative humidity x saturation vapour pressure (Pa)
R_d = 287.058 J/(kg·K) (specific gas constant, dry air)
R_v = 461.495 J/(kg·K) (specific gas constant, water vapour)
T = temperature in Kelvin

The saturation vapour pressure (p_sat) is calculated using the Buck equation:

p_sat = 611.21 x exp((18.678 - T_C/234.5) x (T_C/(257.14 + T_C)))

where T_C is the temperature in Celsius. This gives p_sat in Pascals and is accurate to within 0.02% for temperatures from -40 °C to 50 °C.

Worked Example

At 15 °C and 101,325 Pa with 0% relative humidity:

T = 15 + 273.15 = 288.15 K
p_d = 101,325 Pa (all dry air at 0% RH)
rho = 101,325 / (287.058 x 288.15) = 101,325 / 82,715.8 = 1.2250 kg/m³

This matches the International Standard Atmosphere (ISA) sea-level value of 1.225 kg/m³ exactly.

What Affects Air Density?

Factor	Effect on density	Why
Higher temperature	Lower density	Molecules move faster and occupy more volume
Higher pressure	Higher density	Molecules are compressed into less space
Higher humidity	Lower density	Light water vapour displaces heavier nitrogen and oxygen
Greater altitude	Lower density	Atmospheric pressure decreases with height

Practical Applications

Air density matters in many fields:

Aviation: Aircraft performance (lift, thrust, engine power) all depend on air density. Pilots use density altitude to account for the combined effect of temperature and pressure altitude.
Meteorology: Weather forecasting models track air density to simulate atmospheric dynamics, cloud formation, and storm development.
Wind energy: Wind turbine power output is proportional to air density. A turbine at altitude or on a hot day produces less power than at sea level in cold conditions.
Sports: Thinner (less dense) air at high altitude means less aerodynamic drag. Athletes and cyclists at altitude benefit from reduced drag but face lower oxygen availability.
HVAC engineering: Air conditioning and ventilation systems are sized based on the mass flow rate of air, which depends on density.
Combustion engines: Less dense air means less oxygen per intake stroke, reducing maximum engine power output.

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Sources and method: ISO 2533:1975 Standard Atmosphere; ICAO Doc 7488/3 Manual of the ICAO Standard Atmosphere; Buck, A. L. (1981) "New equations for computing vapour pressure and enhancement factor", Journal of Applied Meteorology 20:1527-1532; Picard, A. et al. (2008) "Revised formula for the density of moist air", Metrologia 45:149-155.

This calculator uses the ideal gas law and the Buck equation for saturation vapour pressure. Results are accurate for typical atmospheric conditions between -40 °C and 60 °C and pressures from 10,000 Pa to 200,000 Pa. For very high humidity near the saturation point, small rounding differences may appear. This calculator is for educational and engineering estimation purposes.