The measurement of the rate of population growth has attracted considerable attention in scientific literature of recent years. This is, no doubt, a result of the continual decline in the birth-rate which has formed the topic of innumerable articles in the popular press and elsewhere. Several attempts have been made to obtain a simple statistical measure of the reproductivity of a population at a particular time—that is, a measure of the extent to which a population will be replacing itself if current fertility and mortality continue indefinitely. It is the aim of this paper,

• in section 1, to discuss the simple approximate formulae that have been suggested;

• in section 2, to discuss some more elaborate and more efficient formulae;

• in section 3, to analyse the effect on the formulae of section 2 of a change in the proportions married at a given age;

• in section 4, to outline the male versus female rate anomaly;

• in section 5, to suggest a formula which avoids the anomaly; and finally,

• in section 6, to discuss the application of these formulae to Australian population statistics.