An investigation conducted with a coking and non-coking coal, shows that grindability of the coals is determined by the grindabilities of its constituent specific gravity fractions and the weight percentage distribution of these same fractions. Though the grindability of the constituent specific gravity fractions of both the coals varies in a systematic manner, certain differences, however, can be observed in the variation pattern between the two coals. It is easier to grind the heaviest fraction of the coking coal than some of its neighbouring heavy fractions. The observed variation indicates the possibility of accurate estimation of the grindability of both the coals studied from routine sink-float data on the basis of the empirical relationship between cumulative mean specific gravity and HGI of cumulative floats at successive specific gravities.