A laboratory based study of tumbling mills has been undertaken in which the mill speed, grate open area and the position of the apertures were systematically varied and their effect on the relationship between water flowrate and hold-up measured. It was found that flow occurred in two distinct zones. One was associated with flow through the grinding media and the other through the pool which developed at higher flowrates in the toe region of the charge. Equations were developed which described the relationship between hold-up and flowrate in each of these zones. These equations were then calibrated using a range of data from pilot and full scale mills where the equilibrium slurry contents and flowrate had been accurately measured. Further validation was then conducted by using sets of independent data. The resultant equations were found to predict the full scale plant data well, covering a range of mill diameters up to 9.6 m and including open as well as closed circuit operations. It is expected that the equations will be of use to plant/equipment designers and researchers interested in modelling the behaviour of autogenous and semi-autogenous mills.