In the calculation of the coefficients of compression models for powders either the initial volume or the particle density is introduced as a normalising factor. The influence of these normalising factors is, however, widely different on coefficients derived from the Kawakita, Walker and Heckel equations. The problems are illustrated by investigations on compaction profiles of 17 materials with different molecular structures and particle densities. It is shown that the particle density of materials with covalent bonds in the Heckel model acts as a key parameter with a dominating influence on the derived apparent yield pressures. Substances with ionic bonds or macromolecular materials with amorphous structures do not show this dependency on the particle density. The influence of density is demonstrated by performing non-linear regression on the Heckel equation where the optimal particle density is estimated. The a parameter in the Kawakita equation is not influenced to a greater degree by variation in the initial volume.