Pharmaceutics
Compressibility Index Calculator
Calculates Carr's compressibility index, the Hausner ratio and bulk and tapped density from a tapped-cylinder test, and describes the powder's flow.
Carr's index
Enter the powder mass, initial volume and tapped volume to see the flow properties.
Powder properties
Accurately weighed; USP uses 25–100 g.
Unsettled volume, read before tapping.
After standard tapping (e.g., 500–1250 taps). Smaller than V₀.
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Flowability classification reference
| Carr's Index (%) | Hausner Ratio | Flow Character | Typical Example |
|---|---|---|---|
| <10 | 1.00–1.11 | Excellent | Glidants, free-flowing granules |
| 11–15 | 1.12–1.18 | Good | Lactose, MCC |
| 16–20 | 1.19–1.25 | Fair | Starch, some APIs |
| 21–25 | 1.26–1.34 | Passable | Fine powders, cohesive blends |
| 26–31 | 1.35–1.45 | Poor | Very cohesive, need granulation |
| >31 | >1.45 | Very Poor | Extremely cohesive, must granulate |
Carr's index scale
Hausner ratio scale
CI = (V₀ − Vₜ) / V₀ × 100
HR = V₀ / Vₜ
ρ_b = m / V₀ · ρ_t = m / Vₜ
m = powder mass (g), V₀ = initial (bulk) volume (mL), Vₜ = tapped volume (mL), ρ_b = bulk density, ρ_t = tapped density.
Because the mass is the same before and after tapping, Carr's index and the Hausner ratio can be worked out from the two volumes alone — the mass is only needed for the densities.
Keep learning
References
- Aulton ME, Taylor KMG, editors. Aulton's Pharmaceutics: The Design and Manufacture of Medicines. 6th ed. Edinburgh: Elsevier; 2022.
- Sinko PJ, editor. Martin's Physical Pharmacy and Pharmaceutical Sciences. 8th ed. Philadelphia: Wolters Kluwer; 2023.
Common questions
Yes. Carr's index, Carr's compressibility index and the compressibility index all refer to (V₀ − Vₜ) / V₀ × 100, which equals (ρ_t − ρ_b) / ρ_t × 100.
USP method I taps 10, 500 and 1250 times and continues until the volume change between readings is small (under 2 mL). Use the same number of taps for every sample you want to compare.
Cylinder volumes are usually read to the nearest 0.5–1 mL, so more decimals would suggest precision the test does not have. The flow band is decided on the unrounded value, so 14.99% counts as Good even though it displays as 15.0%.
Add a glidant such as colloidal silicon dioxide, or granulate the powder to make larger, rounder particles. Re-test afterwards to confirm the improvement.
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