van der Waals' equation for calculating the pressure of a non ideal gas is

$\left(\mathrm{P}+\frac{{\mathrm{an}}^2}{{\mathrm{V}}^2}\right)(\mathrm{V}-\mathrm{nb})=\mathrm{nRT}$

van der Waals' suggested that the pressure exerted by an ideal gas, P_ideal, is related to the experimentally measured pressure, P_real by the equation

$\begin{array}{cccc}{\mathrm{P}}_{\text{ideal }}& ={\mathrm{P}}_{\text{real }}& +& \frac{{\mathrm{an}}^2}{{\mathrm{V}}^2}\\ & \uparrow & & \underset{\stackrel{ }{ }}{\uparrow }\\ & \mathrm{observed}& & \mathrm{correction} \\ & \mathrm{pressure}& & \mathrm{term}\\ & & & \end{array}$

Constant 'a' is measure of intermolecular interaction between gaseous  molecules that gives rise to non-ideal behavior. It depends upon how 
frequently any two molecules approach each other closely. Another correction concerns the volume occupied by the gas molecules. In the ideal gas equation, V represents the volume of the container. However, each molecule does occupy a finite, although small, intrinsic volume, so the effective volume of the gas becomes (V - nb), where n is the number of moles of the gas and b is a constant. The term nb represents the volume occupied by gas particles present in n moles of the gas. Having taken into account the corrections for pressure and volume, we can rewrite the ideal gas equation as follow

$\begin{array}{cc}\left(\mathrm{P}+\frac{{\mathrm{an}}^2}{{\mathrm{V}}^2}\right)& (\mathrm{V}-\mathrm{nb})=\mathrm{nRT}\\ \text{ corrected }& \text{ corrected }\\ \text{ pressure }& \text{ volume }\end{array}$