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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 pharmacologist Charles Jelleff Carr (1910–2005). It measures the relative significance of interparticle interactions.

The Carr index is calculated by the formula $C=100{\frac {V_{T}-V_{B}}{V_{T}}}$, where $V_{B}$ is the volume that a given mass of power 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 $C=100\times (1-{\frac {\rho _{B}}{\rho _{T}}})$, 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. 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 bigger. A Carr index greater than 25 is considered to be an indication of poor flowability, and below 15, of good flowability.

Another way to measure the flow of a powder is the Hausner ratio, which can be expressed as $H=\rho _{T}/\rho _{B}$.

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.

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