Ideal Gas Law

Written by Jerry Ratzlaff on . Posted in Thermodynamics

The ideal gas law can be used to predict pressure, temperature & volume changes in ideal gasses.  An ideal gas is defined as one in which all collisions between atoms or molecules are perfectly elastic and in which there are no intermolecular attractive forces.  In the real world, the ideal gas law must be corrected but it can serve as a good approximation for initial calculations.

Ideal Gas Law formula

\(\large{ p \; V = n \; R \; T }\) 
Symbol English Metric
\(\large{ p }\) = pressure of gas \(\large{\frac{lbf}{in^2}}\)  \(\large{Pa}\) 
\(\large{ \rho }\) (Greek symbol rho) = density \(\large{\frac{lbm}{ft^3}}\) \(\large{\frac{kg}{m^3}}\)
\(\large{ n }\) = number of moles of gas \(\large{dimensionless}\)
\(\large{ R }\) = specific gas constant (gas constant) \(\large{\frac{ft-lbf}{lbm-R}}\) \(\large{\frac{J}{kg-K}}\)
\(\large{ T }\) = temperature \(\large{ R }\) \(\large{ K  }\)
\(\large{ V }\) = volume \(\large{ in^3 }\) \(\large{ mm^3 }\)



  • Temperature is always the absolute temperature which will be either Rankine or Kelvin. For values that are in Fahrenheit, the conversion is °F - 459.67 = R. For values that are in Celsius the conversion is °C - 273 = K.
  • Pressure is absolute pressure, not gauge pressure.
  • Make sure units match on the universal gas constant.


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Tags: Gas Equations Ideal Gas Equations Gas Laws Equations Laws of Physics