The washability data on coals of different origins have been analysed in terms of relation between the cumulative weight fraction of feed material floated and the recovery of non-ash and ash material in the float fraction at a given specific gravity. It has been noted that a unique recovery curve exists for both the recoveries of non-ash and ash material when plotted against the cumulative weight fraction of feed material floated. On the basis of this observation, a simple ＇＇Index of Washability＇＇ has been developed. An explicit mathematical expression has been derived to estimate the index value for a given coal.