Carr index

The Carr index is an indication of the compressibility of a powder. It is calculated by the formula $C=100{\frac {V_{B}-V_{T}}{V_{B}}}$, where $V_{B}$ is the freely settled volume of a given mass of powder, and $V_{T}$ is the tapped volume of the same mass of powder. It can also be expressed as $C=100\times (1-{\frac {\rho _{B}}{\rho _{T}}})$, where $\rho _{B}$ is the freely settled bulk density of the powder, and $\rho _{T}$ is the tapped bulk density of the powder.

The Carr index is frequently used in pharmaceutics as an indication of the flowability of a powder. A Carr index greater than 25 is considered to be an indication of poor flowability, and below 15, of good flowability.

The Carr index is related to the Hausner ratio, another indication of flowability, by the formula $C=100\times (1-1/H)$.

Both the Hausner ratio and the Carr index are sometimes criticized, despite their relationships to flowability being established empirically, as not having a strong theoretical basis. Use of these measures persists, however, because the equipment required to perform the analysis is relatively cheap and the technique is easy to learn.

References

Mark Gibson (2001). Pharmaceutical Preformulation and Formulation: A Practical Guide from Candidate Drug Selection to Commercial Dosage Form. Boca Raton: CRC Press. ISBN 1-57491-120-1.