No steam or gas ever drives anything until it is confined. No Niagara is ever turned into light and power until it is tunneled. No life ever grows until it is focused, dedicated, disciplined.  — Harry Emerson Fosdick


The need to predict gas properties at other states is necessary for engineers to estimate conditions such as pressure drop or vessel capacity. For example, you may know the density of air at standard temperature and pressure (STP), but what about air at 600 °F when it is flowing through your pipe design? Sure, you can go find a table, but if you want to know where that data came from or need a different value without interpolating, the ideal gas equation is here to help.

In general, you would use ideal conditions for an estimate. Gases do not always behave this way and you can get a pretty good explanation from here.

Wikipedia has a good definition and explanation of the base equation here.

After some manipulation, pressure is defined as the following:

When the pressure is held constant between two states, label the initial state with the subscript “known” and leave the final state with no subscript, then divide:

After the constant terms cancel out, the equation is rearranged for density. Now you can get the final density at any temperature where the initial density and temperature are known. For air, I use the initial density of 0.08069 lb/cubic ft at 273.15° K. Make sure to use temperature on the absolute scale (Kelvin °K or Rankine °R) for these calculations, then convert to your desired units.


Interesting observation I noticed while working for the space industry: Most gasses will cool down when you expand them through an orifice or other significant pressure drop component like a pressure reducing valve. If the pressure drop across the component is high enough, then condensate ice will form on and near the exterior of the component from the ambient moist air surrounding it. Helium actually does the opposite. It will warm up instead!


You can pop this small equation in your spreadsheet for future use or use mine for free at the link below. I’ve also added a short video below for further explanation.


Download free spreadsheet for Ideal Gas density calculation here:


*See the disclaimer tab in the spreadsheet concerning liability.

Hope you are able to get some use from the spreadsheet. Feel free to manipulate it to suit your needs.


Best Regards,

David Reeves

The Engineering Job Link

Leave a comment below or send me a separate note.

One thought on “Blowing Hot Air- Ideal Gas

  1. Great internet site! It looks extremely good! Sustain the great job!

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