About the Air Density Calculator
This air density calculator works out how many kilograms one cubic metre of air weighs under the conditions you enter. It treats air as a mixture of dry air and water vapour, each behaving as an ideal gas, so it accounts for the three things that change density: temperature, pressure and humidity. Enter a measured station pressure, or let the calculator estimate pressure from your altitude using the standard atmosphere.
Air density matters wherever air pushes on something or flows through something. Pilots and drone operators use it for lift and take-off performance, cyclists and engineers for aerodynamic drag, HVAC designers for fan and duct sizing, and sports scientists to explain why balls fly further at altitude. Physics students use it to check the ideal-gas law with real numbers.
A useful surprise: humid air is less dense than dry air at the same temperature and pressure, because a water molecule (18 g/mol) is lighter than the nitrogen and oxygen it replaces (about 29 g/mol on average). Standard sea-level air at 15 °C and 1013.25 hPa is 1.225 kg/m³. The altitude option uses the troposphere model (valid to 11 km) and ignores weather systems, so a real barometer reading is more accurate when you have one.
With the default inputs, the air density is 1.1988 kg/m³. Change any value above to recalculate instantly.
How to use the air density calculator
- 1Choose Celsius or Fahrenheit and enter the air temperature.
- 2Enter the relative humidity (0% for dry air).
- 3Enter a barometric station pressure, or switch to altitude to estimate it.
- 4Read the density in kg/m³ and lb/ft³ and compare it with standard air.
- 5Use the chart to see how the same air would change with temperature.
Formula and method
Moist air is treated as two ideal gases sharing the same volume. The water vapour partial pressure p_v is the relative humidity times the saturation vapour pressure, calculated with the Magnus formula using the Alduchov–Eskridge coefficients (accurate to about 0.4% between −40 °C and 50 °C). The dry-air partial pressure is what is left of the total pressure, and each part is divided by its own specific gas constant times the absolute temperature.
When you choose altitude instead of pressure, the calculator uses the International Standard Atmosphere troposphere equation p = 101,325 × (1 − 2.25577×10⁻⁵·h)^5.25588 Pa, which assumes a standard temperature lapse rate. Real pressure varies with the weather by a few percent, so a barometer reading gives a better answer.
- ρ
- Density of moist air (kg/m³)
- p_d, p_v
- Partial pressures of dry air and water vapour (Pa)
- R_d
- Specific gas constant of dry air, 287.058 J/(kg·K)
- R_v
- Specific gas constant of water vapour, 461.495 J/(kg·K)
- T, t
- Temperature in kelvin and in °C
- RH
- Relative humidity as a fraction
Worked examples
Standard sea-level air
Dry air at 15 °C (288.15 K) and 101,325 Pa: ρ = 101,325 ÷ (287.058 × 288.15) = 1.225 kg/m³, the value used as "standard air" in aviation and engineering.
Hot, humid summer day at sea level
At 30 °C the saturation vapour pressure is about 42.4 hPa, so 80% humidity gives 33.9 hPa of water vapour. Dry air would be 1.1644 kg/m³; swapping in the lighter vapour drops it to about 1.150 kg/m³, roughly 6% thinner than standard air.
Denver (5,280 ft) on a mild day
The standard atmosphere puts pressure at 1,609 m near 834 hPa. At 20 °C and 30% humidity the air density is about 0.988 kg/m³, almost 20% below sea-level standard, which is why baseballs carry further at Coors Field.
Frequently asked questions
What is the density of air at sea level?+
Standard sea-level air at 15 °C and 1013.25 hPa has a density of 1.225 kg/m³ (0.0765 lb/ft³). On a real day it typically ranges from about 1.15 kg/m³ when hot and humid to over 1.3 kg/m³ in cold winter air.
Is humid air heavier or lighter than dry air?+
Lighter. Water vapour has a molar mass of about 18 g/mol, while dry air averages about 29 g/mol. Adding vapour at the same pressure and temperature replaces heavier molecules with lighter ones, so humid air is slightly less dense.
How does altitude affect air density?+
Pressure falls with height, and density falls with it. Air density is about 1.11 kg/m³ at 1,000 m, 0.91 kg/m³ at 3,000 m and roughly 0.74 kg/m³ at 5,000 m in the standard atmosphere, before any temperature differences.
What pressure should I enter?+
Use the actual (station) pressure where you are. Weather reports usually quote pressure corrected to sea level, which is far too high for a location at altitude; if you only have that, use the altitude option instead.
What is density altitude?+
Density altitude is the altitude in the standard atmosphere that has the same air density as current conditions. Hot, humid days make density altitude higher than the real elevation, which reduces aircraft lift and engine power.