An empirical compressibility index based on changes in powder packing between loose bulk and tapped measurements.
Also called: compressibility index; Carr compressibility index.
How is Carr index calculated?
Use loose bulk density and tapped density for the same material and defined measurement conditions, expressed in the same units. The index reports the relative packing change as a percentage; it is not a mass or a flow rate.
Carr index (%) = 100 × (tapped density − bulk density) / tapped density
Hausner ratio divides tapped density by bulk density. With the example above it is 1.25, rather than 20%. Both calculations use the same two densities, so reporting both is not two independent flow experiments.
Check whether an index is reported as a percentage or a fraction before comparing numbers. A result of 20% corresponds to 0.20 as a fraction, not 20 in a fractional convention.
A smaller index means a smaller relative difference between the loose and tapped densities in that measurement. Descriptive flow grades can help communicate an empirical classification, but a grade is not a universal capsule-filling acceptance limit.
Report the underlying densities, sample condition and method with the index. Hopper discharge and fill-weight consistency require process evidence beyond this packing measurement.