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In the phenomenologic relation for the coercivity (Hc) of a powder Hc = A + B(1-P) one generally takes for the volume packing factor P, the "macroscopic" value, which is calculated from the macroscopic dimensions of the sample and the weight. A series of powder with varying pigment volume packing factors were made by densifying the original powder. Applying for these series the Hc vs P relation we have found that for P, the "microscopic" value must be taken. This means that the Hc is primarily controlled by the volume packing factor of the "microscopic" clusters or aggregates. We also found that this "microscopic" P, can easily be measured by means of mercury intrusion porosimetry.