This post was inspired by the fact that my daughter got ahold of the scissors last night. I wondered about hair, and how it works, how fast it grows etc., because she cut her hair down to the scalp in some spots
Encyclopedia Brittanica explains it this way:
"The number of scalp hairs, which grow about 0.5 in. (13 mm) per month, averages 100,000–150,000. The hair shaft (above the skin) is dead tissue, composed of keratin. Only a few growing cells at the base of the root are alive."
SOOOO, my daughter basically has a shaved head right now, and a rate of 1/2 an inch a month, to get back to where her hair was, it's going to take about a year and a half.
hair. (2012). In Britannica Concise Encyclopedia.