This paper discusses a procedure for the development of design fires for buildings and presents a study undertaken to quantify design fires for commercial buildings. This procedure includes building surveys, full-scale experiments and computer modelling. In this study, a survey of commercial premises was conducted to determine fire loads and types of combustibles present in these buildings. Based on the results of the survey a number of fuel packages were designed that represent fire loads and combustible materials in commercial buildings. The fuel packages were used to perform full-scale, post-flashover fire tests to collect data on heat release rates, compartment temperatures and production and concentration of toxic gases. Results from these experiments are presented and discussed. The paper also discusses a computer modelling study in which a computational fluid dynamics model was used to simulate the tests and to model a fire in a real-scale commercial store.