It is imperative to provide adequate quantity and quality of space for all captive animals. Yet practically all guidelines on the housing of primates in the laboratory specify minimum cage sizes based solely on body weight. We argue that no single factor, such as body weight, is sufficient to determine cage size. Instead a suit of characteristics should be used that include morphometric, physiological, ecological, locomotor, social reproductive, and behavioural characteristics. Ideally, the primate's age, sex, and individual history should also be taken into account. In this paper, we compare this suite of characteristics for some commonly used primates whose weights overlap, to illustrate important differences amongst them. For good animal welfare and good quality science it is necessary to be sensitive to such species differences when determining suitable cage sizes.