Given a placement of wireless nodes in space and a traffic demand between pairs of nodes, can these traffic demands be supported by the resulting network? A key issue for answering this question is wireless interference between neighbouring nodes including self interference along multi-hop paths. This paper presents a generic model for sustainable network load in a multi-hop wireless network under interference constraints, and recasts this model into a multicommodity flow problem with interference constraints. Using Farkas’ Lemma, we obtain a necessary and sufficient condition for feasibility of this multicommodity flow problem, leading to a tight upper bound on network throughput. Our results are illustrated by examples.