Air Density Calculator
The density of air at a given temperature, pressure and relative humidity — dry and moist — with the water-vapour pressure, the density altitude, and how the figure compares with standard sea-level air.
Air is a mixture of dry air and water vapour, each behaving as an ideal gas.
How the air density calculator works
Air is a mixture of dry air and water vapour, each behaving as an ideal gas. Dry air density is pressure over the gas constant times absolute temperature; humid air is lighter, because water molecules are lighter than nitrogen and oxygen. The vapour pressure comes from the Magnus formula at the dew point the humidity implies. Standard sea-level air is 1.225 kg/m³, and the density altitude is the height in the standard atmosphere where air is as thin as yours.
Formula: ρ = p_d / (R_d T) + p_v / (R_v T); p_v = RH × 6.1078 × 10^(7.5t/(t+237.3)) hPa
Worked examples
| Inputs | Air density (kg/m³) | Note |
|---|---|---|
| 25 °C, sea level, 60% humidity | 1.1755 | about 1.18 kg/m³ |
| Standard conditions | 1.22498 | 1.225 |
| A hot, humid day at altitude | 0.99823 | thin air |
FAQFrequently asked questions
What is the density of air?
About 1.225 kg/m³ at 15 °C and sea-level pressure with no humidity — the standard atmosphere figure. Warmer, wetter or higher air is lighter; cold, dry, high-pressure air is heavier.
Why does humidity make air lighter?
Because a water molecule (mass 18) is lighter than the nitrogen (28) and oxygen (32) it displaces. The effect is small — a few grams per cubic metre — but real, and it matters to aircraft and engines.
What is density altitude?
The height in the standard atmosphere where air would be as thin as it is where you are. Pilots use it because aircraft performance depends on density, not the altimeter reading; a hot day at a low airfield can have the density altitude of a mountain strip.
Where does the formula come from?
The ideal gas law applied separately to dry air and water vapour, with their own gas constants, and added. The Magnus formula gives the saturation vapour pressure at the temperature.
What pressure should I enter?
The actual station pressure, not the sea-level-corrected figure weather reports give. At 500 m elevation the real pressure is about 955 hPa on a standard day.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Physical Laboratory — the UK's national measurement institute
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.