tapped density

Tapped density is how dense a powder becomes after you settle it by tapping. We all do this instinctively: pour cereal into a bowl and shake or tap it, and the flakes drop down so the bowl looks less full. The particles rearrange to fill the air gaps, and the same mass now occupies a smaller volume.

It is measured by placing loose powder in a graduated cylinder and mechanically tapping it a set number of times (often hundreds to a few thousand) until the volume stops shrinking. The mass divided by that final, smallest volume is the tapped density. Because the voids have partly collapsed, it is always higher than the bulk density of the same powder.

The interest is rarely in the tapped density alone but in how far the volume changed on tapping. A powder that barely consolidates was already well packed and flows freely, whereas one whose volume drops a lot held many open voids that only rearranged under agitation, a sign of cohesive, poorly flowing material.

Standardisation matters because the result depends on the tapping apparatus, drop height and number of taps, all fixed by pharmacopoeial methods. The bulk and tapped densities together feed directly into the Carr compressibility index and the Hausner ratio.