Van der Waal's equation for one mol of a gas is given as
$\left[\mathrm{P}+\frac{\mathrm{a}}{{\mathrm{V}}^2}\right]\left(\mathrm{V}-\mathrm{b}\right)=\mathrm{R}\mathrm{T}$
Where b is volume correction. It arises due to finite size of gaseous molecules.The constant ‘b’ is a measure of volume occupied by the molecules. It is actually four times of molar volume of gas molecule.