Capacity is an important property for QoS support in Mobile Ad Hoc Networks (MANETs) and has been extensively studied. However, most approaches rely on simplified models (e.g., protocol interference, unidirectional links, perfect scheduling or perfect routing) and either provide asymptotic bounds or are based on integer linear programming solvers. In this paper, we present a probabilistic approach to capacity calculation by linking the normalized throughput of a communicating pair in an ad hoc network to the connection probability of the two nodes in a socalled schedule graph GT