  We propose a simple method to estimate the central charge of the conformal
field theory corresponding to a critical point of a two-dimensional lattice
model from Monte Carlo simulations. The main idea is to use the Wang-Landau
flat-histogram algorithm, which allows us to obtain the free energy of a
lattice model on a torus as a function of torus radii. The central charge is
calculated with a good precision from a free energy scaling at the critical
point. We apply the method to the Ising, tricritical Ising (Blume-Capel), Potts
and site-diluted Ising models, and also discuss estimation of conformal
weights.
