A thermodynamic approach to deriving an equation of state has been elaborated on the basis of excluded volume. A master differential equation relating pressure to excluded volume has been derived whose particular solutions in the zero and first approximation are the famous Planck and van der Waals equations, respectively. The second approximation yields a new more accurate equation of state covering a wider state range from rarefied gas to the critical state. At the same time, the equation maintains the original simplicity that characterizes the Planck and van derWaals equations. (C) 2003 American Institute of Physics.