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 density 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 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 $$C=100\times(H-1)$$.