Bulk and tapped density testing predicts how a powder will behave in hoppers, feeders and tablet press dies. A weighed sample is poured into a graduated cylinder and its loose volume is read. The cylinder is then tapped mechanically until the volume stops changing, and the final volume gives the tapped density. The method is described in USP <616> and the equivalent Ph. Eur. chapter.
From the two densities, the calculator gives Carr’s Index (compressibility index) and the Hausner ratio. Both describe how much the powder packs down under tapping, and therefore how freely it flows.
A 50 g powder sample occupies 100 mL before tapping and 80 mL after tapping.
Both values fall in the “fair” band, so the powder will flow but may need a glidant or a different granulation method to run well at speed.
There is no single pass or fail limit, because flow requirements depend on the process. The widely used scale is:
| Carr’s Index | Hausner ratio | Flow character |
|---|---|---|
| 1–10% | 1.00–1.11 | Excellent |
| 11–15% | 1.12–1.18 | Good |
| 16–20% | 1.19–1.25 | Fair |
| 21–25% | 1.26–1.34 | Passable |
| 26–31% | 1.35–1.45 | Poor |
| 32–37% | 1.46–1.59 | Very poor |
| Above 38% | Above 1.60 | Very, very poor |
For direct compression, values below about 15% are usually preferred. Above 25% the formulation often needs changes. Your own specification for a blend or granule, set from development data, overrides the general scale. The two indices are linked, so they should always agree with each other.
Use the same cylinder size, sample mass and tapping settings every time a given material is tested, and record them with the result. Density values from different set-ups are not directly comparable, so a trend only means something when the method is held constant.
Sample the material the way it will be used. Test the blend after lubrication if flow in the press is the question, because magnesium stearate can change both the bulk and tapped volumes. Test granules after drying and milling, and let them reach room humidity first, since moisture strongly affects flow.
Use the results alongside other indicators such as angle of repose, flow rate through an orifice and the actual behaviour in the hopper. A single index rarely tells the whole story, but a large change in Carr's Index between two trial batches is a reliable early warning that something in the formulation or process has shifted.
The checks on this page relate to the documents below. Standards are revised from time to time, so confirm the current edition and follow your own approved procedures.
Below about 15% suggests good flow. Above about 25% suggests poor flow that may need formulation changes.
Bulk density is measured on the loosely poured powder. Tapped density is measured after mechanical tapping compacts it.
They use the same two densities and describe the same behaviour on different scales. Carr’s Index is a percentage and the Hausner ratio is a ratio.
Poor flow is a common cause of weight variation, but the density result is only an early indicator. Confirm with in-process weight checks.
Related tools and guides: Carr’s Index and Hausner Ratio Explained · Mixer Capacity Calculator · Weight Variation Calculator
By Sajjad Ahmad. For reference and education only; verify limits against your SOPs and the current pharmacopoeia. See the methodology.