A guide to cortisol and every way of testing it. Information, not medical advice. Read more
haircortisoltest

Tools

Which months does a hair sample cover?

Scalp hair grows about 1 cm a month, so each centimetre from the scalp is roughly a month further back. Enter the date you will cut your sample and its length to see which months it holds.

Most laboratories test the 3 cm nearest the scalp.

Want to see the effect of a change?

A new job, a stressful period ending, a treatment or a new sleep routine.

Why the dates are approximate

Scalp hair grows by 0.6 to 1.4 cm a month, about 1 cm on average, so a 3 cm sample can hold anything from about two to five months of growth.1 Hair also takes about two weeks to reach the surface of the scalp, and scissors leave a stub of about 0.8 cm behind, so the centimetre nearest the scalp reflects hair formed roughly 1.3 to 2.2 months before the cut.2 The calculator allows for that lag; the result is still an estimate.

Why most tests use 3 cm

Three centimetres covers about three months, long enough to smooth out the ups and downs of any single week, and almost all published studies use segments of 3 cm or less.3 Hair further from the scalp has been washed and exposed to sunlight for longer, and its cortisol content falls from one segment to the next; researchers suggest a look-back of about six months at most.4 Samples are taken from the back of the head, just below the crown, where results vary least between neighbouring samples.5 See how to take a hair sample and month-by-month (segmental) analysis.

Sources

  1. Adav SS & Ng KW (2025). The multifaceted role of hair as a biospecimen: recent advances in precision medicine and forensic science. Exp Mol Med pubmed.ncbi.nlm.nih.gov
  2. LeBeau MA et al. (2011). The role of variations in growth rate and sample collection on interpreting results of segmental analyses of hair. Forensic Sci Int pubmed.ncbi.nlm.nih.gov
  3. Martins-Silva T et al. (2025). Hair cortisol measurement: a systematic review of current practices and a proposed checklist for reporting standards. Psychoneuroendocrinology pubmed.ncbi.nlm.nih.gov
  4. Kirschbaum C et al. (2009). Hair as a retrospective calendar of cortisol production. Psychoneuroendocrinology pubmed.ncbi.nlm.nih.gov
  5. Sauvé B et al. (2007). Measurement of cortisol in human hair as a biomarker of systemic exposure. Clin Invest Med pubmed.ncbi.nlm.nih.gov