Relative density or specific gravity of the coal depends on the rank of the coal and degree of mineral impurity. Knowledge of the density of each coal play is necessary to determine the properties of composites and blends. The density of the coal seam is necessary for conversion of resources into reserves.
Relative density is normally determined by the loss of a sample's weight in water. This is best achieved using finely ground coal, as bulk samples are quite porous.  To determine in-place coal tonnages however, it is important to preserve the void space when measuring the specific gravity.