The relationship between an external stress applied to a bulk and its resulting porosity—i.e. its compaction behavior—is studied in this work, varying the median particle diameter by using different batches of carbonyl iron powder (CIP) as well as by mixing them. Firstly, compaction equations of other authors and an algebraic relation suggested by us are presented. Secondly, these descriptions are compared to data obtained by compacting CIP. Last, for powder mixtures, the role of the particles’ median and mean diameters is studied in experiments and DEM-simulations, respectively. We show that they take on the role of the relevant length scale appearing in the algebraic compaction law.