The advantages that the rooftop rainwater harvesting system has as a source of water supply have been examined. The observed daily rainfall records of 10 years and the current total roof area of the facilities at Mbeya University of Science and Technology as the catchment area were used. Using a water balance model to determine the suitable water use that will cover 100% of the time, the model indicated that for the current roof area a water supply of 120 lts per day can be met when a storage tank whose capacity is 13.5 m³ is installed. When values higher that 120 l/day are simulated a tank of higher capacity is required to meet the water demand. The study recommends on the necessity of installing rooftop rainwater harvesting system so as to increase the water supply reliability and reduction of cost. The selection of a suitable storage tank capacity should take into consideration the future development plans so as to reduce the construction cost of new storage tank.