Moist Air - Partial Pressure and Daltons Law

Calculate the total pressure of moist air using Daltons Law of partial pressures. The total pressure is the sum of the partial pressures of dry air and water vapor.

Daltons Law of Partial Pressure

Daltons Law states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the constituent gases.

The partial pressure is the pressure each gas would exert if it alone occupied the volume of the mixture.

Formula

p = pa + pw

Where:

  • p = total pressure of air (Pa, N/m²)
  • pa = partial pressure of dry air (Pa, N/m²)
  • pw = partial pressure of water vapor (Pa, N/m²)

Alternative Formula

p = ρa × 286.9 × T + ρw × 455 × T

Where:

  • ρa = density of air (kg/m³)
  • ρw = density of water vapor (kg/m³)
  • T = temperature (K)
Calculator

Inputs

Results

Total Pressure (Method 1)-- Pa
Total Pressure (Method 2)-- Pa
ParameterSymbolUnitDescription
Total PressurepPa (N/m²)Total pressure of moist air
Partial Pressure of Dry AirpaPa (N/m²)Pressure exerted by dry air component
Partial Pressure of Water VaporpwPa (N/m²)Pressure exerted by water vapor component
Density of Airρakg/m³Mass density of dry air
Density of Water Vaporρwkg/m³Mass density of water vapor
TemperatureTKAbsolute temperature
Gas Constant for AirRaJ/(kg·K)286.9 J/(kg·K)
Gas Constant for Water VaporRwJ/(kg·K)455 J/(kg·K)