Background: Children and adolescents with mental health problems have poor service cover in low- and middle-income countries. Little is known about the resources that would be required to provide child and adolescent mental health services (CAMHS) in these countries. The purpose of this study was to calculate the human resources and associated costs required to scale up CAMHS in South Africa. Methods: A spreadsheet model was developed to calculate mental health service resources, based on an estimation of the need for services in a given population. The model can be adapted to specific settings by adjusting population size, age distribution, prevalence, comorbidity, levels of coverage, service utilisation rates, workloads, length of consultations and staff profile. Steps in the modelling include population identification; estimates of prevalence, service utilisation and staffing; and costing. Results: Using a nominal total population of 100,000 (of which 43,170 would be children and adolescents under 20 years of age), the following full-time equivalent staff are required at minimum coverage level: 5.8 in PHC facilities, 0.6 in general hospital outpatient departments (OPDs), 0.1 in general hospital inpatient facilities, 1.1 in specialist CAMHS OPDs, 0.6 in specialist CAMHS inpatient facilities, 0.5 in specialist CAMHS day services, and 0.8 in regional CAMHS teams. This translates into roughly $21.50 and $5.99 per child or adolescent per annum nationally for the full coverage and minimum coverage scenarios respectively. When comparing the results of this model with current realities in South Africa, there remains a substantial shortfall in existing levels of CAMHS provision. Conclusions: The model can be used as an advocacy tool to engage with planners and policy makers on a rational basis. It can also be adapted for use in other countries, and is intended to support wider calls for a global scaling up of mental health services.