bulk density

Bulk density is the weight of a loosely poured powder divided by the space it takes up, voids and all. Think of gently filling a measuring cup with flour without packing it: the cup holds a certain mass, but much of its volume is actually air between the grains. Bulk density counts that trapped air as part of the volume.

It is measured by pouring a known mass of powder into a graduated cylinder without tapping or compacting, then reading the volume it occupies. Because the volume includes the spaces between particles, the value is lower than the powder's true (solid) density. The gap between the two reflects how much void the loose powder contains.

Bulk density tells a formulator practical things: whether a given dose will fit into a capsule shell of a chosen size, how big a tablet will be, how much a drum of material will weigh, and how the powder will behave when fed to a machine. Two powders of the same chemistry can differ greatly in bulk density depending on particle size, shape and how they pack.

On its own bulk density is descriptive rather than predictive of flow, but compared with the tapped density it becomes powerful: the change between the two underlies the Carr index and Hausner ratio, the standard quick measures of flowability.