Objective. To develop a model for estimating the services and human resources needed to care for people with severe psychiatric conditions in a hypothetical population of 100000 people in South Africa. Method. Annual mental health service needs were estimated in terms of numbers of daily patient visits (DPV) in ambulatory care, the number of beds required, and staffing. Developed within a spreadsheet format, the model allows for the adjustment of key service variables according to estimated or existing service data. Results. At 100% coverage, 87 DPV, 28 acute beds, and 10 medium-long stay beds are necessary for a population of 100000 people. This would require 35.2 full-time equivalent mental health staff: 21.3 for inpatient care, 12.0 for ambulatory care, and 1.9 for management. Conclusion. Because the model can produce a range of service recommendations, the assumptions that inform it should be dearly stated and justified. This method makes the assumptions on which services are planned explicit acid allows for a rational approach to decision making.