Hausner ratio

The Hausner ratio is simply the tapped density divided by the bulk density, a number telling you how much denser a powder gets once it is settled by tapping. Because tapping can only pack particles closer, the ratio is always at least one; the further above one it climbs, the more the loose powder was holding open spaces, and the worse it tends to flow.

Like Carr's index it captures the same physical idea, that cohesive powders consolidate a lot and free-flowing ones barely do, but expresses it as a ratio rather than a percentage. The two are mathematically linked, so they always agree, and a lab will usually report one or the other from the very same density measurement.

Interpretation follows pharmacopoeial bands: a ratio of 1.00 to 1.11 indicates excellent flow, 1.12 to 1.18 good, 1.19 to 1.25 fair, 1.26 to 1.34 passable, 1.35 to 1.45 poor, 1.46 to 1.59 very poor, and above 1.60 extremely poor. These guide decisions about glidants, granulation, and machine settings.

The same caveats apply: the ratio is comparative rather than absolute, depends on standardised measurement, and reflects flow tendency without fully predicting behaviour in a specific hopper or press. It is most useful as a quick early screen and as part of a fuller flow assessment.