Hair density is usually expressed as hairs per square centimetre. It is used in clinical studies, trichoscopy, phototrichograms and transplant planning. Low density can result from shedding, miniaturisation, scarring loss or naturally lower baseline density. Density alone does not tell the whole story because hair calibre, curl, colour contrast and styling strongly affect visible coverage. In treatment monitoring, consistent measurement methods matter more than one isolated number.
DermNet trichoscopy: https://dermnetnz.org/topics/trichoscopy ; Gupta et al., relative efficacy network meta-analysis, JAMA Dermatology 2022