Air density calculator
Density of moist air from temperature, pressure and humidity.
Fill in the fields and the result will appear here automatically.
Estimate a dry-air/water-vapour mixture density from actual temperature, absolute pressure and relative humidity. At the same temperature and pressure, replacing dry air with water vapour lowers density. Aircraft take-off distance and engine thrust are not calculated.
How it works
Formula and logic
es=6.1078·10^(7.5 t/(t+237.3)) hPa; e=es·RH/100. Add partial densities: ρ=100(p −e)/(287.058·T)+100 e/(461.495·T), T=t+273.15 K. The factor 100 converts hPa to Pa.
Example
20 °C, 1013.25 hPa and 50% give ρ≈1.1988 kg/m³, dry air≈1.2041 kg/m³ and e≈11.690 hPa. The mixture is about 2.136% below the 1.225 kg/m³ reference.
Fields and units
- Temperature — °C
- Atmospheric pressure — hPa
- Relative humidity — %
How to use
- — Enter local absolute pressure, rather than a weather report’s sea-level reduction.
- — Compare moist and dry densities at the same p and t.
- — The 1.225 kg/m³ reference is dry air; its deviation is not an aerodynamic acceptance limit.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- OU CAPS: liquid-water Tetens coefficients
- Limitation
- Ideal gas mixture without aerosols. Tetens approximates saturation over liquid water, not ice; no universal error bound is claimed. The branch t>−237.3 °C avoids the singularity and is an algebraic bound, not a validated accuracy range. Require p>0, RH 0–100% and e≤p.
FAQ
Why is moist air lighter than dry air?
Because a water molecule is lighter than an average air molecule: 18 against 29 atomic units. At the same pressure and temperature a cubic metre holds the same number of molecules, so swapping heavy ones for light ones lowers the mass.
How far does density fall in the heat?
Estimate a dry-air/water-vapour mixture density from actual temperature, absolute pressure and relative humidity. At the same temperature and pressure, replacing dry air with water vapour lowers density. Aircraft take-off distance and engine thrust are not calculated. 20 °C, 1013.25 hPa and 50% give ρ≈1.1988 kg/m³, dry air≈1.2041 kg/m³ and e≈11.690 hPa. The mixture is about 2.136% below the 1.225 kg/m³ reference.
What is the standard 1.225 figure?
The density of dry air at 15 °C and 1013.25 hPa, the reference of the International Standard Atmosphere. Aerodynamic figures are normalised to it so that tests in different weather stay comparable.
Does altitude matter?
Enter local absolute pressure, rather than a weather report’s sea-level reduction.