What is the Density of Air?
Air density (denoted by the Greek letter ρ, rho) measures the mass of air per unit volume (kg/m³ or lb/ft³). For dry air at sea level at 59 °F (15 °C) and standard atmospheric pressure of 1013.25 hPa (14.696 psi), the air density is approximately 1.225 kg/m³ (0.0765 lb/ft³).
Air density changes with temperature, pressure, relative humidity, and altitude. As a rule of thumb, density drops by about 0.035–0.036 kg/m³ per 1,000 ft (300 m) of altitude gain.
How to Calculate Air Density
The air density calculator uses the ideal gas law extended to moist air. The total density of humid air is the sum of the density of dry air and the density of water vapour:
ρ = p_d / (R_d × T) + p_v / (R_v × T)
Where:
- ρ — air density (kg/m³)
- p_d — partial pressure of dry air = p − p_v (Pa)
- p_v — vapour pressure = φ × e_s(T) (Pa)
- R_d = 287.058 J/(kg·K) — specific gas constant for dry air
- R_v = 461.495 J/(kg·K) — specific gas constant for water vapour
- T — absolute temperature in Kelvin (T = T_°C + 273.15)
- φ — relative humidity (0–1)
- e_s(T) — saturation vapour pressure (Buck equation)
Air Density Definition — What Is the Air Density Formula?
For dry air (humidity = 0 %), the formula simplifies to:
ρ_dry = p / (R_d × T)
Moist air is less dense than dry air at the same temperature and pressure because water vapour molecules (M = 18 g/mol) are lighter than the average dry-air molecule (M ≈ 29 g/mol) they displace.
Saturation Vapour Pressure (Buck Equation)
The calculator uses the Buck equation to find the saturation vapour pressure e_s:
e_s = 611.21 × exp[(18.678 − T_°C / 234.5) × T_°C / (257.14 + T_°C)] (Pa)
Air Density Table — Density of Dry Air
The table below shows standard dry-air density at sea-level pressure (1013.25 hPa) for various temperatures.
| Temperature | °C | Density (kg/m³) | Density (lb/ft³) |
|---|---|---|---|
| −40 °F | −40 | 1.514 | 0.0945 |
| 14 °F | −10 | 1.341 | 0.0837 |
| 32 °F | 0 | 1.293 | 0.0807 |
| 59 °F (standard) | 15 | 1.225 | 0.0765 |
| 68 °F | 20 | 1.204 | 0.0752 |
| 86 °F | 30 | 1.164 | 0.0727 |
| 104 °F | 40 | 1.127 | 0.0703 |
| 122 °F | 50 | 1.093 | 0.0682 |
| 212 °F | 100 | 0.946 | 0.0591 |
Density of Air in kg/m³ and in English Units (lb/ft³)
The SI unit of air density is kg/m³. The imperial/US equivalent is lb/ft³ (pounds per cubic foot). Conversion: 1 kg/m³ = 0.062428 lb/ft³.
Standard Air Density
The International Standard Atmosphere (ISA) defines standard sea-level conditions as:
- Temperature: 15 °C (59 °F)
- Pressure: 1013.25 hPa (29.921 inHg)
- Humidity: 0 % (dry air)
- Air density: 1.225 kg/m³ (0.0765 lb/ft³)
This value is used as a baseline in aeronautics, wind-energy calculations, and atmospheric modelling. The calculator shows the computed density as a percentage of the standard value.
What Is Air Pressure?
Air pressure is the force exerted by the weight of the air column above a given point. At sea level this is approximately 101 325 Pa (1013.25 hPa / mbar), also known as 1 atm or 14.696 psi. Pressure decreases with altitude — roughly halving every 5.5 km (18 000 ft). The standard atmosphere formula used in this calculator is:
p = 101 325 × (1 − 2.25577 × 10⁻⁵ × h)^5.2561 [h in metres]
What Is Relative Humidity?
Relative humidity (RH) is the ratio of the actual water-vapour pressure to the saturation vapour pressure at the same temperature, expressed as a percentage. RH = 100 % means the air is fully saturated and any additional moisture will condense. Higher humidity lowers air density because water vapour is lighter than the nitrogen and oxygen it replaces.
What Is Dew Point?
The dew point is the temperature at which the air must be cooled (at constant pressure) for water vapour to start condensing. It is calculated using the Magnus formula:
T_dew = 243.04 × γ / (17.625 − γ) γ = ln(RH/100) + 17.625 × T / (243.04 + T) [T in °C]
A dew point close to the actual temperature indicates high relative humidity and a feeling of "mugginess"; a large difference indicates dry air.
Frequently Asked Questions
- Does warm air have higher or lower density than cold air?
- Lower. Warmer molecules move faster and occupy more space, so the same volume holds less mass.
- Why does humid air have lower density than dry air?
- Water vapour (molar mass 18 g/mol) displaces heavier nitrogen (28 g/mol) and oxygen (32 g/mol) molecules, reducing average molecular mass and thus density.
- How does altitude affect air density?
- Both pressure and (usually) temperature drop with altitude, so density decreases. The ISA formula predicts a drop of roughly 0.035 kg/m³ per 1,000 ft (305 m).
- What is the density of air at sea level in English units?
- 0.0765 lb/ft³ (at 59 °F, 29.92 inHg, dry air) — the ISA standard.
- Where is air density used in engineering?
- Aerodynamic drag, lift calculations (aviation), wind-turbine power output, HVAC system design, and combustion-air calculations all depend on local air density.