Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

Moist Air - Partial Pressure and Daltons Law

The pressure in a mixture of dry air and water vapor - humid or moist air - can be estimated by using Daltons Law of partial pressures.

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.

Daltons Law of partial pressure for moist air can be expressed as

p = pa + pw                            (1)


p = total pressure of air (Pa, N/m2)

pa = partial pressure dry air (Pa, N/m2)

pw = partial pressure water vapor (Pa, N/m2)

With the partial pressure for dry air and water vapor (1) can be modified to:

p = ρa286.9T + ρw455T                                    (2)


ρa = density of air (kg/m3)

ρw = density of water vapor (kg/m3)

Related Topics

  • Air Psychrometrics

    Moist and humid air calculations. Psychrometric charts and Mollier diagrams. Air-condition systems temperatures, absolute and relative humidities and moisture content in air.

Related Documents


Search is the most efficient way to navigate the Engineering ToolBox.

Engineering ToolBox - SketchUp Extension - Online 3D modeling!

3D Engineering ToolBox Extension to SketchUp - add parametric components to your SketchUp model

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with older versions of the amazing SketchUp Make and the newer "up to date" SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp Make/Pro from the Extension Warehouse !

Translate this Page

Translate this page to Your Own Language .

About the Engineering ToolBox!

Privacy Policy

We don't collect information from our users. More about

We use a third-party to provide monetization technologies for our site. You can review their privacy and cookie policy here.

You can change your privacy settings by clicking the following button: .


This page can be cited as

  • The Engineering ToolBox (2004). Moist Air - Partial Pressure and Daltons Law. [online] Available at: [Accessed Day Month Year].

Modify the access date according your visit.

3D Engineering ToolBox - draw and model technical applications! 2D Engineering ToolBox - create and share online diagram drawing templates! Engineering ToolBox Apps - mobile online and offline engineering applications!

Unit Converter