In many industrial applications a major part of the data on size distributions uses screening to define size, whereas an important part of the size distribution information is determined using sub-sieve size analysis. It is necessary to have an accurate method of converting one type of data to the other. A technique is presented for conversion of Sedigraph size distribution data to equivalent screen size. The technique consists of first characterizing the material by determining the Sedigraph size distribution on a sample of the powder carefully wet-screened to lie between 270 and 400 mesh (53-38 mu m) and fitting the data by a log-normal or log-logistic distribution function to give mu and sigma or mu and lambda values characteristic of the material. The Sedigraph size distribution of a sample of less than 400 mesh powder screened from the total sample can then be converted to an equivalent screen size distribution by a back-calculation technique, using the characteristic parameters of the material in a constrained search program. The technique is illustrated by application to data taken around an industrial grinding mill.