Carr index

The Carr index (also: Carr's index or Carr's Compressibility Index) is an indication of the compressibility of a powder. It is named after the scientist Ralph J. Carr, Jr.

The Carr index is calculated by the formula $C=100{\frac {V_{B}-V_{T}}{V_{B}}}$, where $V_{B}$ is the volume that a given mass of powder would occupy if let settled freely, and $V_{T}$ is the volume of the same mass of powder would occupy after "tapping down". It can also be expressed as $C=100(1-\rho _{B}/\rho _{T})$, 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. In a free-flowing powder, the bulk density and tapped density would be close in value, therefore, the Carr index would be small. On the other hand, in a poor-flowing powder where there are greater interparticle interactions, the difference between the bulk and tapped density observed would be greater, therefore, the Carr index would be larger. A Carr index greater than 25 is considered to be an indication of poor flowability, and below 15, of good flowability.<ref>{{cite book |author1=Kanig, Joseph L. |author2=Lachman, Leon |author3=Lieberman, Herbert A. |title=The Theory and Pra

General Bibliography

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.