Regular haircut
Human scalp hair grows on average about one eighth inch per week or one half inch per month. Most clipper guards are numbered in eighths of an inch. The number of the guard denotes the number of weeks of hair growth left on the scalp when a clipper with a certain numbered guard is guided over the head with the guard in contact with the scalp. A #1 guard leaves 1/8 inch (3 mm), one week's growth of hair, on the scalp; a #2 guard leaves 2/8" (6.3 mm), two weeks' hair growth, on the scalp; a #3 guard leaves 3/8" (9 mm), three weeks' hair growth on the scalp; and so on.
Underarm hair
Underarm hair, as human body hair, normally starts to appear at the beginning of puberty, with growth usually completed by the end of the teenage years.
Axillary hair goes through 4 stages of development, driven by weak androgens produced by the adrenal in males and females during adrenarche, and testosterone from the testicle in males during puberty. Like Tanner Staging for pubic hair, axillary hair can be staged according to the staging system, named for Pediatric Endocrinologist, , as follows:
Staging a patient's axillary hair will allow the physician to track the child's development longitudinally.