Air Density Calculator
Work out the density of moist air (kg/m³) from pressure, temperature and relative humidity. It compares your result against the standard sea-level value of 1.225 kg/m³, so you can see how hot, humid or high-altitude conditions thin the air.
Enter Values
How to use this calculator
- Enter the air pressure in hPa (leave it at the 1013.25 default for standard sea level).
- Enter the air temperature in °C — negative values are allowed for cold conditions.
- Optionally enter the relative humidity (0–100%); leave it at 0 for dry air, then read the density, the percentage difference from standard, and the temperature in kelvin.
How it works
Air is treated as a mix of dry air and water vapour. The saturation vapour pressure is found with the Magnus/Tetens formula SVP = 6.112 × exp(17.62·T / (243.12 + T)); the actual vapour pressure Pv scales that by the relative humidity, and the dry-air partial pressure Pd is the total pressure minus Pv. Each gas is added with its own specific gas constant — 287.05 J/(kg·K) for dry air and 461.495 J/(kg·K) for water vapour — giving ρ = Pd/(287.05·Tk) + Pv/(461.495·Tk).
Worked example
Worked example. At standard sea level — 1013.25 hPa, 15 °C, 0% humidity — the vapour pressure is zero, so ρ = 101,325 / (287.05 × 288.15) ≈ 1.225 kg/m³, exactly the standard reference. Warm it to 35 °C at 80% humidity and the density falls to about 1.126 kg/m³, roughly 8% thinner.
Common mistakes
- Entering pressure in the wrong unit — this tool expects hectopascals (hPa), which equal millibars; 1 atm ≈ 1013.25 hPa.
- Assuming humid air is heavier — water vapour is lighter than dry air, so higher humidity lowers density.
- Setting relative humidity outside 0–100% — values above 100% are unphysical and rejected.
Frequently asked questions
Why is humid air less dense than dry air?
A water molecule (18 g/mol) is lighter than the average air molecule (~29 g/mol). At a fixed pressure and temperature, adding water vapour displaces heavier nitrogen and oxygen, so the mixture weighs less per cubic metre.
What is density altitude?
It is the altitude at which the standard atmosphere has the same density as your actual conditions. Hot, humid, low-pressure air behaves like a higher altitude — reducing aircraft lift and engine power — which is why pilots watch density, not just elevation.
Related tools
- Cloud Base Height Calculator
- Wind Speed at Height Calculator
- Beaufort Wind Scale Calculator
- Dew Point Calculator
- Absolute Humidity Calculator
- Apparent Temperature Calculator
Explore more in Weather & Environmental Monitoring.
Tip: Enter any known values to calculate the remaining results.
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