Service-area specific optimization is a necessity for large-scale wireless networks. For this task, a model to estimate the quality of a radio network configuration is required. In the case of cellular radio networks, much work exists on models for Radio Network Planning (RNP). This work cannot be applied to optimize IEEE 802.11-based Wireless Mesh Networks (WMNs), as their multi-hop structure and the distributed medium access deviates significantly from cellular networks. In this paper, we present a model for IEEE 802.11-based WMNs that fulfills the following criteria: (i) It has low complexity, enabling iterative optimization; (ii) it has sufficient accuracy in comparison to simulation; (iii) it includes a physical layer model that takes into account interference and multiple transmission rates.