Carr's compressibility index

Carr's compressibility index is a simple percentage that says how much a powder shrinks when tapped, used as a thumbnail measure of how well it flows. The idea is intuitive: a free-flowing powder is already packed snugly and has little room to consolidate, while a sticky, cohesive powder is full of trapped voids and collapses noticeably when shaken down.

It is calculated as one hundred times the tapped density minus the bulk density, divided by the tapped density, which is the same as the percentage drop in volume on tapping. A value of 100 times (V0 minus Vf) over V0 using the loose and tapped volumes gives the identical number. Low percentages mean good flow; high percentages mean poor flow.

The pharmacopoeial scale runs roughly: 1 to 10 percent is excellent flow, 11 to 15 good, 16 to 20 fair, 21 to 25 passable, 26 to 31 poor, 32 to 37 very poor, and above 38 extremely poor. These bands let a formulator decide whether a glidant is needed or whether the powder must be granulated before tableting.

Despite its convenience, the index is an indirect, comparative tool, not a fundamental property. It is sensitive to how the densities were measured, and it correlates with flow rather than defining it, so it is best used alongside the Hausner ratio, angle of repose and direct flow measurements.