There is increasing recognition that parity of esteem between mental and physical health disorders is essential to improve the course, outcome and quality of life of individuals within different populations. Achieving this parity now underpins the objectives of several nations. Here, we argue that parity of esteem between mental and physical health can only be realized when parity of esteem also exists across mental health disorders, particularly in terms of service commissioning and planning. Using first-episode psychosis and early intervention in psychosis services as a motivating example, we demonstrate how carefully conducted psychiatric epidemiology can be translated to develop precise forecasts of the anticipated incidence of first-episode psychosis in different populations, based on an understanding of underlying local needs and inequalities. Open-access prediction tools such as PsyMaptic will allow commissioners of mental health services to more effectively allocate resources across services, based on empirical evidence and local need, thus reducing inequalities in access to mental health care.