# What is the air density?

Air density calculator: the density of dry or humid air from the temperature, the air pressure and the relative humidity or dew point, in kg/m³ and lb/ft³, with the vapour and dry-air pressures.

- Page: https://www.acalculator.org/physics/air-density-calculator
- JSON spec: https://www.acalculator.org/physics/air-density-calculator.json
- Version: 6c44f368ff72

## Default answer

Example with the default inputs (Air temperature 58.9999999999999 °F, Air pressure 29.9212555797485 inHg, Humidity Relative humidity, Relative humidity 50%): The air density is 1.2211 kg/m³ (0.07623 lb/ft³).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| t | Air temperature | The temperature of the air. |
| p | Air pressure | The actual (station) air pressure, not the sea-level altimeter setting. |
| hum | Humidity | Give the humidity as relative humidity or a dew point, or treat the air as dry. |
| rh | Relative humidity | The relative humidity, from 0% to 100%. |
| td | Dew point | The dew point of the air; it cannot be above the air temperature. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| density | Air density (kg/m³) | ρ = p_d ÷ (R_d T) + p_v ÷ (R_v T), in kilograms per cubic metre. |
| lbft3 | Air density (lb/ft³) | The air density in pounds per cubic foot. |
| ratio | Of standard sea-level air (%) | The density as a percentage of 1.225 kg/m³, the standard sea-level value. |
| dry | Dry air at the same T and p (kg/m³) | p ÷ (R_d T): the density if the air held no water vapour. |
| vapour | Vapour pressure (hPa) | The partial pressure of water vapour, p_v. |
| dryPressure | Dry-air pressure (hPa) | p_d = p − p_v. |
| saturation | Saturation vapour pressure (hPa) | The Tetens saturation vapour pressure at the air temperature. |

## Method

p_sat(t) = 610.78 × exp(17.27 t ÷ (t + 237.3)) Pa (t in °C); p_v = RH × p_sat(T), or p_sat(dew point), or 0; p_d = p − p_v; ρ = p_d ÷ (287.058 × T) + p_v ÷ (461.495 × T), T in kelvins.

## Assumptions

- Moist air is a mix of ideal gases: dry air and water vapour.
- The pressure is the actual air pressure where the air is, not an altimeter setting reduced to sea level.

## Worked examples

1. t = 288.15, p = 101,325, hum = dry gives density = 1.224978. Source: Wikipedia, Density of air (ρ = p_d ÷ (R_d T) + p_v ÷ (R_v T); R_d = 287.058, R_v = 461.495 J/(kg·K); Tetens p_sat = 0.61078 exp(17.27 (T − 273.15) ÷ (T − 35.85)) kPa), https://en.wikipedia.org/wiki/Density_of_air (retrieved 2026-10-05).
2. t = 303.15, p = 101,325, hum = rh, rh = 100% gives saturation = 42.429261, density = 1.145936. Source: Wikipedia, Density of air (ρ = p_d ÷ (R_d T) + p_v ÷ (R_v T); R_d = 287.058, R_v = 461.495 J/(kg·K); Tetens p_sat = 0.61078 exp(17.27 (T − 273.15) ÷ (T − 35.85)) kPa), https://en.wikipedia.org/wiki/Density_of_air (retrieved 2026-10-05).
3. t = 293.15, p = 90,000, hum = dew, td = 283.15 gives vapour = 12.279224, density = 1.06399. Source: Wikipedia, Density of air (ρ = p_d ÷ (R_d T) + p_v ÷ (R_v T); R_d = 287.058, R_v = 461.495 J/(kg·K); Tetens p_sat = 0.61078 exp(17.27 (T − 273.15) ÷ (T − 35.85)) kPa), https://en.wikipedia.org/wiki/Density_of_air (retrieved 2026-10-05).

## FAQ

### How do you calculate air density?

For dry air, ρ = p ÷ (R_d × T) with R_d = 287.058 J/(kg·K) and T in kelvins. At 1,013.25 hPa and 15 °C: 101,325 ÷ (287.058 × 288.15) = 1.225 kg/m³. For humid air, split the pressure into dry air and water vapour and add the two densities.

### What is the density of air at sea level?

The standard value is 1.225 kg/m³ (0.0765 lb/ft³), for dry air at 15 °C and 1,013.25 hPa.

### Does humidity make air lighter?

Yes. A water molecule (18 g/mol) is lighter than the average air molecule (about 29 g/mol), so at the same temperature and pressure humid air is less dense. Saturated air at 30 °C is about 1.146 kg/m³, against 1.164 kg/m³ dry.

### How does temperature change air density?

At a fixed pressure, density is inversely proportional to the absolute temperature: warming air from 0 °C (273 K) to 30 °C (303 K) lowers its density by about 10%.

### Should I use the altimeter setting as the pressure?

No. The altimeter setting is reduced to sea level. Use the actual station pressure where the air is; at altitude it is much lower.

### What saturation vapour pressure formula is used?

The Tetens formula: 6.1078 × exp(17.27 t ÷ (t + 237.3)) hPa, with t in °C. Wikipedia gives the same formula with T in kelvins.

## Sources

- Wikipedia, Density of air: ρ = p_d ÷ (R_d T) + p_v ÷ (R_v T), R_d = 287.058 and R_v = 461.495 J/(kg·K), and the Tetens saturation vapour pressure, retrieved 2026-10-05. https://en.wikipedia.org/wiki/Density_of_air
- NIST Special Publication 811 (2008), Appendix B.8: 1 lb/ft³ = 16.018 46 kg/m³, 1 inHg = 3,386.389 Pa, retrieved 2026-10-05. https://www.nist.gov/pml/special-publication-811