A method is developed to calculate the required number of subjects k in a reliability study, where reliability is measured using the intraclass correlation ρ. The method is based on a functional approximation to earlier exact results. The approximation is shown to have excellent agreement with the exact results and one can use it easily without intensive numerical computation. Optimal design configurations are also discussed; for reliability values of about 40 per cent or higher, use of two or three observations per subject will minimize the total number of observations required. © 1998 John Wiley & Sons, Ltd.