A Wireless Sensor Network (WSN) consists of several sensor nodes with limited energy source. Lifetime of such network depends on the residual energy of its nodes. However, to successfully achieve the designated mission, WSNs should meet a required lifetime. Accordingly, computing lifetime allows a better prediction of the network availability. Therefore, this paper proposes an analytical model to predict the lifetime of IEEE 802.15.4 wireless sensor networks. Our proposed model is based on a complete energy consumption analysis. In fact, it considers the most important sources of energy consumption such as overhearing, idle listening, overheads and collisions due to interference. Compared to simulation results and other analytical approaches, our model gives a reliable lifetime prediction for a given sensor network scenario.