Air Density Calculator

What Is Air Density?

Air density is the mass of air per unit volume, usually expressed in kg/m³. It depends on temperature, pressure, and humidity. Warmer air is less dense; higher pressure increases density; water vapor is lighter than dry air, so humid air is slightly less dense at the same temperature and pressure.

Air Density Formula

$$\rho = \frac{P - 0.378\,e}{R_d\,T}$$

where $P$ is total pressure (Pa), $e$ is actual vapor pressure (Pa), $R_d = 287.05$ J/(kg·K) is the specific gas constant for dry air, and $T$ is absolute temperature (K). The factor 0.378 accounts for the lower molecular weight of water vapor.

Vapor pressure comes from relative humidity: $e = \text{RH} \times e_s$, with saturation pressure $e_s$ from the Magnus formula. At 20 °C and 1013.25 hPa with 50% RH, density is about 1.20 kg/m³.

Frequently Asked Questions

Why does humidity lower air density?

Water molecules (M ≈ 18 g/mol) are lighter than nitrogen and oxygen (M ≈ 28 g/mol). At fixed temperature and pressure, moist air displaces heavier molecules, so density decreases slightly.

What units should I use?

Enter temperature in °C, pressure in hPa (millibars), and relative humidity in percent. The calculator converts internally to SI units.

How does air density affect flight?

Lower density reduces lift and engine thrust. Aircraft performance drops on hot, humid days and at high altitude where air is thinner.

Is this the same as the ideal gas law?

Yes, with a humidity correction. Dry air follows $\rho = P / (R_d T)$. Moist air uses the virtual-pressure correction shown above.

What is typical sea-level air density?

At standard conditions (15 °C, 1013.25 hPa, dry air), density is about 1.225 kg/m³. Humidity and temperature changes shift this value.

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What Is Air Density?

Air Density Formula

Frequently Asked Questions

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