Bulk Density and Powder Particle Size

In the case of equal sized spherical particles packed on a cubic grid, the bulk density is independent of particle size (neglecting boundary effects).

e.g. 1 sphere in a 1 x 1 x 1 cubic container: volume of sphere (= bulk density) ≈ 0.524

For 8 spheres, volume per sphere x 8 ≈ 0.524

The same goes for 27, 64, 125 etc.

The more variation in particle size there is for a given sample, the higher the bulk density will be, as the smaller particles will tend to fill in the interstices between the larger ones.