The power consumed by a combinational circuit is dictated by the switching activities of all signals associated with the circuit. An analytical approach is proposed for calculating signal activities for combinational circuits. The approach is based on a Markov chain signal model, and directly accounts for correlations present among the signals. The accuracy of the approach is verified by comparing signal activity values calculated using the proposed approach with corresponding values produced through simulation studies. It is also demonstrated that the proposed approach is computationally efficient.