By integrating queuing theory and compartmental models of flow we demonstrate how changing admission rates, length of stay and bed allocation influence bed occupancy, emptiness and rejection in departments of geriatric medicine. By extending the model to include waiting beds, we show how the provision of extra, emergency use, unstaffed, back up beds could improve performance while controlling costs. The model is applicable to all lengths of stay, admission rates and bed allocations. The results show why 10–15% bed emptiness is necessary to maintain service efficiency and demonstrate how unstaffed beds can serve to provide a more responsive and cost effective service. Further work is needed to test the validity and applicability of the model.