NB-IoT has been defined in 3GPP release 13. It modifies LTE radio access procedures and protocols to fulfill energy consumption, low data rate and latency requirements associated with IoT applications. One of the major issues of NB-IoT deployment is the design of performance evaluation models that make it possible to plan and dimension this new system. This work proposes a new analytical model based on stochastic geometry. It provides a closed form expression of the successful transmission of an NB-IoT system in the uplink for a given sensor spatial intensity. The accuracy of the proposed analytical model is validated by simulation.