In this paper we propose a discrete time Markov model to assess the improvement that may be achieved using opportunistic routing. We show that our model can be used to compute the probability distribution and moments of the number of transmissions needed to send a packet from the source to the destination in a variety of scenarios. We use our model to compute some opportunistic routing performance measures in a network with a shadowing radio prop- agation model with different network topologies, and varying the maximum number of candidates used by the opportunistic routing algorithm. Our model can be easily extended to evaluate other aspects of opportunistic routing not covered in this paper. Therefore, we believe that our model yields a simple, yet flexible and powerful tool to evaluate opportunistic routing protocols.