A methodology for the interpolation of washabilily data is presented. This procedure provides a means whereby the washability data for both the weight and the attributes (e.g., ash, sulfur, etc.) can be interpolated for any size and/or relative density interval desired. Unlike some techniques which rely on the interpolation of interval values, this technique involves the interpolation of cumulative values. Since point values are used, there is no ambiguity as to what size or relative density corresponds to which interval value.