A method for calculating the short-range order part of the free energy of order - disorder systems is proposed. The method is based on the application of the cumulant expansion to the exact configurational entropy. Second-order correlation corrections to the mean-field approximation for the free energy are calculated for arbitrary thermodynamic phases and types of interaction. The resulting quadratic approximation for the correlation entropy leads to substantially better values of transition temperatures for the nearest-neighbour cubic Ising ferromagnets.