We investigate a non solvable two-dimensional ferromagnetic Ising model with nearest neighbor plus weak finite range interactions. We rigorously establish one of the predictions of Conformal Field Theory (CFT), namely the fact that at the critical temperature the finite size corrections to the free energy are universal. The corresponding central charge, defined in terms of the coefficient of the first subleading term to the free energy, is constant and equal to 1/2. Work in collaboration with A. Giuliani.