Air Density Calculator

Calculate the density of moist air from temperature, barometric pressure, and relative humidity using the ideal gas law.

Quick Facts

Standard atmosphere
1.225 kg/m³ at sea level
ISA reference: 15°C, 1013.25 hPa, dry air (0% humidity).
Humidity effect
Moist air is less dense than dry air
Water vapor (18 g/mol) is lighter than the N₂/O₂ mix it displaces (~29 g/mol).

Your Results

Calculated
Air density
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Kilograms per cubic meter (kg/m³)
Air density (imperial)
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Pounds per cubic foot (lb/ft³)
Specific volume
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Volume per unit mass, 1/ρ (m³/kg)
Vs. standard atmosphere
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Difference from ISA sea level (1.225 kg/m³)

Ready

Enter temperature, pressure, and humidity, then press Calculate.

About the Air Density Calculator

Air density is the mass of air per unit volume, and it changes constantly with temperature, barometric pressure, and humidity. It matters for aircraft performance and density-altitude corrections, HVAC and combustion calculations, wind-turbine power output, ballistics, drone flight time, and any fluid-dynamics or aerodynamics problem where drag and lift scale directly with how dense the air is. This calculator applies the ideal gas law to the dry-air and water-vapor components of moist air separately and adds the two contributions together.

The formula

Total air density is the sum of the partial density of dry air and the partial density of water vapor:

ρ = Pd / (Rd × T) + Pv / (Rv × T)

  • ρ — air density (kg/m³)
  • Pd — partial pressure of dry air (Pa) = total pressure minus water-vapor pressure
  • Pv — partial pressure of water vapor (Pa) = relative humidity × saturation vapor pressure
  • T — temperature in kelvin (°C + 273.15)
  • Rd — specific gas constant for dry air, 287.058 J/(kg·K)
  • Rv — specific gas constant for water vapor, 461.495 J/(kg·K)

Saturation vapor pressure is estimated with the Tetens formula, es = 6.1078 × 10^(7.5T / (T + 237.3)) hPa, where T is temperature in °C. Multiplying es by the relative humidity (as a fraction) gives the actual vapor pressure Pv; subtracting Pv from the total barometric pressure gives the dry-air pressure Pd. At 15°C, 1013.25 hPa, and 0% humidity, the formula returns 1.225 kg/m³ — the International Standard Atmosphere (ISA) sea-level value used as this calculator's reference point.

Working with the inputs

  • Use the actual barometric (station) pressure at your location, not pressure "corrected" to sea level — station pressure already reflects your altitude, since pressure drops roughly exponentially as elevation increases.
  • Temperature is entered in Celsius; the calculator converts internally to kelvin for the gas-law terms.
  • Relative humidity of 0% models perfectly dry air; most weather stations report humidity directly, so enter that value as-is.

Knowing the limits

The ideal gas law and the Tetens approximation are accurate for the temperatures, pressures, and humidity levels found in Earth's lower atmosphere. They are not intended for extreme conditions — very high pressure, near-freezing precision work requiring the more complex Arden Buck or CIPM-2007 formulas, or non-atmospheric gas mixtures. For everyday meteorology, aviation, and engineering estimates, the error introduced is negligible.

Frequently Asked Questions

What formula does this calculator use?
It applies the ideal gas law separately to the dry-air and water-vapor components of the atmosphere: density = Pd/(Rd×T) + Pv/(Rv×T), where Pd and Pv are the partial pressures of dry air and water vapor in pascals, T is temperature in kelvin, Rd is 287.058 J/(kg·K), and Rv is 461.495 J/(kg·K). Vapor pressure is derived from relative humidity using the Tetens saturation-vapor-pressure formula.
Why is humid air less dense than dry air?
A water vapor molecule (molar mass about 18 g/mol) is lighter than the nitrogen and oxygen molecules that make up dry air (average about 29 g/mol). At the same temperature and pressure, adding water vapor displaces some of the heavier dry-air molecules, so moist air weighs slightly less per unit volume than dry air — the opposite of the common assumption that humid air feels heavier.
What is standard sea-level air density?
The International Standard Atmosphere (ISA) defines sea-level air density as 1.225 kg/m³ (about 0.0765 lb/ft³), based on 15°C, 1013.25 hPa of pressure, and dry air. This calculator uses that value as the reference point for the percent-difference result.
How much does altitude affect air density?
Air density falls roughly exponentially with altitude because atmospheric pressure drops as there is less air above you. This calculator does not take altitude directly — enter the actual barometric (station) pressure at your location, which already reflects your elevation, along with the local temperature and humidity.

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