It is shown that tricalcium phosphate follows a simple Heckel relationship in pressures of usual tablet compression, only if a density of 1.92 g/mL is used. This coincides with the density obtained by wet pycnometry, whereas the crystallographical density is 3.1 g/mL. The interpretation is that some of the pore space is occluded and does not constitute a part of the Heckel pore space.