How To Calculate How Long You Have Had Head Lice
Learn about the lice cycle to calculate how long you have had head lice.
Feb 21, 2026 · 4 min read
TL;DR
You can estimate infestation length from the nit position: nits more than 1cm from the scalp are usually 2+ weeks old (hair grows ~1cm per month). Eggs hatch in 7–10 days; adult lice live ~30 days.
Understanding the lice cycle
Female lice lay eggs near the scalp. Eggs hatch in 7–10 days. Young lice mature in about 10 days. Adult lice live ~30 days and lay more eggs. So an infestation can grow quickly.
Calculating how long you've had head lice
Nits are laid at the base of the hair. As hair grows ~1cm per month, nits further from the scalp are older. Empty casings mean the louse has hatched. Helps estimate timing but doesn't change treatment — treat as soon as you find lice. Check first.