While a variety of flow measurement devices are available to measure the flow of water through closed pipe systems, these devices generally only function correctly when the pipes are completely full of water. Accurate measurement of water flows in partially-filled pipes is extremely difficult. In siphonic drainage systems, this problem is further compounded by the unsteady flow conditions that occur in the pipework during the priming process. This has been a major obstacle to understanding the performance of these systems in practice. In order to accurately model the priming process in multioutlet siphonic roof drainage systems, a method of estimating the instantaneous flows through the partially-filled individual pipes needs to be developed. This paper describes an experimental method of determining flows in partially-filled pipes using a propeller-type current meter to measure flow velocity and a pressure transducer to measure water depth and a modified version of the continuity equation. A computational model is presented which estimates the unsteady flows passing through partially-filled pipework. Overall, the experimental results are promising and correspond well with the model. The results of this study will ultimately be used to develop an unsteady flow model of the priming process in multi-outlet siphonic roof drainage systems.