Air Density

Free Air Density calculator with clear step-by-step results.

About Air Density

Air density comes from the ideal gas law, not from weighing a sample: ρ = p ÷ (R × T) with absolute temperature in kelvin. Humidity lowers it, which is counter-intuitive — water vapour has a lower molar mass than the nitrogen and oxygen it displaces, so humid air is lighter than dry air at the same pressure and temperature. The calculation therefore splits the total pressure into dry and vapour partial pressures and applies each gas constant separately.

Formulas

Saturation and vapour pressure

Psat = 6.1078 × 10^(7.5T ÷ (T + 237.3)); Pv = Psat × RH ÷ 100

Moist air density

ρ = Pd ÷ (287.058 × T) + Pv ÷ (461.495 × T), pressures in Pa and T in kelvin

Worked example

With the default values below, Air Density returns Air Density of 1.1988. Change any input above and every figure here is recalculated in your browser.

Inputs used

Absolute air pressureminimum 1 hPa: 1,013.25 hPa
Air temperatureaccepts -80–70 °C: 20 °C
Relative humidityaccepts 0–100 %: 50 %

Results

Air Density: 1.1988 kg/m³
Density if the Air Were Dry: 1.2041 kg/m³
Water Vapour Pressure: 11.690 hPa
Relative to Sea-Level Standard (1.225 kg/m³): 97.86%

Step by step

1Values used
2Saturation and vapour pressure
3Moist air density
4Air Density= 1.1988 kg/m³
5Density if the Air Were Dry= 1.2041 kg/m³
6Water Vapour Pressure= 11.690 hPa
7Relative to Sea-Level Standard (1.225 kg/m³)= 97.86

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Uses the Tetens approximation for saturation vapour pressure and treats air as an ideal gas. For metrology, aviation performance or certification work use the full CIPM formulation and calibrated instruments.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Uses the Tetens approximation for saturation vapour pressure and treats air as an ideal gas. For metrology, aviation performance or certification work use the full CIPM formulation and calibrated instruments.

Frequently Asked Questions

Why is humid air less dense?

Water vapour has a molar mass of 18 g/mol against about 29 for dry air. At a given pressure and temperature the total molecule count is fixed, so replacing heavy air molecules with lighter water molecules reduces the mass per cubic metre.

What is standard air density?

1.225 kg/m³ at 15 °C and 1013.25 hPa, the ISA sea-level condition. The final result shows your conditions as a percentage of that, which is the quantity that drives lift, drag and engine output.

Should I use station or sea-level pressure?

Station pressure — the actual absolute pressure where you are. Sea-level-corrected pressure from a weather report will overstate density at altitude, sometimes substantially.