In this paper, a new fast and accurate methodology to evaluate the blocking probability (burst loss probability) in dynamic WDM networks without wavelength conversion is presented. The proposed model considers different traffic loads at each network connection (heterogeneous traffic). To take into account the wavelength continuity constraint, the method sees the network as a sequence of networks where all the links have capacity 1. Every network in the sequence is evaluated separately using an heuristic-based analytical technique. Then, a procedure combines the results of these evaluations in a way that captures the dependencies that occur in real systems due to the competition for bandwidth between the different connections. The method efficiently achieves results very close to those obtained by simulation, but orders of magnitude faster, allowing the evaluation of the blocking probability of all users (connections) for mesh network topologies. We illustrate the use of this procedure in the dimensioning of a WDM network, that is, in calculating the number of wavelengths on every network link, by considering the traffic load of every user, the routing algorithm and the maximum connection blocking probability acceptable by every user (quality of service).