The excluded volume to be a key to deriving an equation of state was shown. Pressure and excluded volume were interrelated as an integral and an integrand, the former being less and the latter more sensitive to errors. Within the frame of the second approximation of the excluded volume theory, a sufficiently accurate three-term equation of state covering a wide density range was derived. Since the criterion of simplicity was used, the appraoch described can be modified by using other criteria.