Projections of the social expenditure to GDP ratio indicate the extent of the 'burden' of population ageing on future workers and have been used by governments to aid policy decisions in such areas as immigration and superannuation. This article shows that the social expenditure to GDP ratio is heavily dependent on assumptions made about real spending growth, productivity growth, unemployment and participation rates. It produces a framework that makes the assumptions underlying the projections clear and enables the results of changing the assumptions to be easily compared. The projected ratios are significantly higher than those obtained in previous Australian studies. Copyright 1993 The University of Melbourne, Melbourne Institute of Applied Economic and Social Research.