Air Density Calculator
Work out the density of moist air from temperature, pressure and relative humidity. Returns the dry-air and water-vapour contributions separately so you can sanity-check the result.
Air density (kg/m³)
1.2
- Dry-air contribution (kg/m³)
- 1.19
- Water-vapour contribution (kg/m³)
- 0.01
- Saturation vapour pressure (Pa)
- 2,338.2
- Vapour partial pressure (Pa)
- 1,169.1
- Dry-air partial pressure (Pa)
- 100,155.9
- Temperature (K)
- 293.15
Humid-air density via the ideal-gas mixture ρ = p_d/(R_d·T) + p_v/(R_v·T), with R_d = 287.058 J/(kg·K), R_v = 461.495 J/(kg·K) and T = 293.15 K. Saturation vapour pressure from the Tetens equation p_sat = 610.78·exp(17.27·T_c/(T_c+237.3)) gives 2338.2 Pa; the partial vapour pressure is RH·p_sat = 1169.1 Pa. The dry-air share carries 1.1902 kg/m³ and the vapour share carries 0.008642 kg/m³, summing to 1.1988 kg/m³. At 15 °C and 101 325 Pa dry, this returns the ICAO standard-atmosphere value 1.2250 kg/m³.
How to use this calculator
Type the air temperature in degrees Celsius, the total atmospheric pressure in hectopascals (hPa, the same as millibar) and the relative humidity as a percentage. The result, in kilograms per cubic metre, updates instantly along with the dry-air and vapour partial densities.
How the calculation works
Moist air is treated as a mixture of two ideal gases. Total density is ρ = p_d / (R_d · T) + p_v / (R_v · T), where R_d = 287.058 J/(kg·K) is the dry-air gas constant, R_v = 461.495 J/(kg·K) is the water-vapour gas constant (both NIST / CIPM-2007), T is absolute temperature in kelvin, p_v is the partial pressure of water vapour and p_d = P − p_v is the dry-air partial pressure. The water-vapour partial pressure comes from p_v = RH · p_sat, with p_sat from the Tetens / Magnus saturation formula p_sat = 610.78 · exp(17.27·T_c / (T_c + 237.3)) Pa, accurate to better than 0.1 % between 0 °C and 60 °C.
Worked example
Take a comfortable room: 20 °C, 1013.25 hPa, 50 % RH. The Tetens formula gives p_sat ≈ 2338.5 Pa, so p_v ≈ 1169.3 Pa and p_d ≈ 100 155.7 Pa. T = 293.15 K. Then ρ_d = 100 155.7 / (287.058 × 293.15) = 1.1902 kg/m³ and ρ_v = 1169.3 / (461.495 × 293.15) = 0.0086 kg/m³, summing to ρ ≈ 1.1988 kg/m³ — matching the value tabulated in the Engineering Toolbox and ASHRAE Handbook to four decimal places. As a second check, dropping RH to 0 % at 15 °C and 101 325 Pa returns ρ = 1.2250 kg/m³, the ICAO standard-atmosphere sea-level density.
Frequently asked questions
What is the density of air at sea level?
The ICAO International Standard Atmosphere defines sea-level air at 15 °C and 101 325 Pa as dry air with density 1.2250 kg/m³ exactly. Real-world humid air at 20 °C, 1013 hPa, 50 % RH is slightly lower — about 1.199 kg/m³ — because water vapour is less dense than the dry-air mixture it displaces.
Why is humid air less dense than dry air?
A water molecule (H₂O, molar mass 18.0 g/mol) is lighter than the average air molecule (~28.96 g/mol). At a given temperature and total pressure, each vapour molecule that joins the mixture displaces a heavier dry-air molecule, so the overall mass per unit volume drops. This is why warm muggy air feels "thin" and why aircraft performance falls in high humidity.
Which formula does this calculator use?
The ideal-gas humid-air mixture: ρ = p_d/(R_d·T) + p_v/(R_v·T). Saturation vapour pressure comes from the Tetens / Magnus equation p_sat = 610.78·exp(17.27·T_c/(T_c+237.3)) Pa. For meteorology and engineering at near-surface conditions this is accurate to roughly 0.1 %; for laboratory-grade work, swap in the full CIPM-2007 formula with compressibility corrections.
What units should I use for pressure?
Hectopascals (hPa) — numerically identical to millibar (mbar). Standard sea-level pressure is 1013.25 hPa = 1013.25 mbar = 1 atm. If your gauge reads inches of mercury, multiply by 33.8639 to get hPa; if it reads kPa, multiply by 10.
Does altitude affect air density?
Yes — and strongly, because pressure falls roughly exponentially with height. To use this calculator at altitude, plug in the local ambient pressure (from a barometer or the station-level METAR), not 1013 hPa. At 2000 m the ambient pressure is about 795 hPa, which alone drops the density by ~21 %.
How does this affect aviation and motorsport?
Lift, drag, thrust, and naturally-aspirated engine power all scale with air density. A hot, humid, high-altitude day can cut take-off thrust by 10–15 % and runway performance by even more — pilots compute "density altitude" (the altitude in the standard atmosphere with the same density). Drag-racing teams adjust jetting and gear ratios based on a similar number, called the "air density grain" or "DA".