Enter the diameter of the medulla and the diameter of the hair into the Medullary Index Calculator. The calculator will evaluate the Medullary Index.
Medullary Index Formula
MI = M / H
- MI = medullary index (unitless ratio, 0 to 1)
- M = medulla diameter (µm)
- H = total hair shaft diameter (µm)
Both diameters must be measured at the same point on the hair shaft and in the same units. The result is dimensionless, so the choice of unit cancels out. Measurements are typically taken from the mid-shaft under a compound microscope at 100x to 400x.
Reference Values
Medullary index is used in forensic and wildlife hair analysis to help separate human from non-human hair.
| Medullary Index | Interpretation |
|---|---|
| < 0.33 | Likely human |
| 0.33 to 0.50 | Overlap zone, inconclusive on its own |
| ≥ 0.50 | Likely non-human (animal) |
| Source | Typical MI Range |
|---|---|
| Human (scalp) | 0.15 to 0.30 |
| Dog | 0.50 to 0.70 |
| Cat | 0.50 to 0.70 |
| Cow / Horse | 0.50 to 0.90 |
| Rabbit / Deer | > 0.80 |
Example
A hair sample shows a medulla diameter of 14 µm and a total shaft diameter of 65 µm.
MI = 14 / 65 = 0.215
The index is below 0.33, which is consistent with human hair. Confirm with scale pattern, pigment distribution, and medulla continuity before reporting a conclusion.
FAQ
Can the medullary index exceed 1.0?
No. The medulla sits inside the cortex, so its diameter cannot be larger than the full shaft.
Where should you measure on the hair?
Use the mid-shaft, away from the root and tip, where the medulla is most consistent.
Is MI alone enough to identify a species?
No. Use it as one of several features. Cuticular scale pattern, medulla type (continuous, fragmented, absent), and pigment distribution are all needed for a defensible identification.
