A number of different models have been used to describe the residence time distributions (RTDs) measured for dry and wet ball milling by various investigators in various countries. It is useful to be able to compare the results between different workers and this requires a knowledge of the equivalence of the models. A method is presented here that permits this comparison for the usual two parameter residence time distribution models, where one parameter is the mean residence time and the other is a characteristic parameter that defines the shape of the residence time distribution. The method defines the best correlation as that which produces the closest agreement of the predicted mill product size distributions over a wide range of values of mean residence time. The correlations are given in graphical form and in equation form. In some circumstances simulation results are not sensitive to the RTD model chosen, provided that the correct match of characteristic parameters is used.