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.
Knemometry
Short-term growth consists of small growth spurts (mini growth spurts). In the human neonate, these spurts occur at intervals of 2 to 10 days, and they reach maximum velocities of up to 0.2 mm per hour at the lower leg. Growth hormone therapies have been shown to significantly alter the dynamics of short-term growth. Catch-up growth (compensatory growth after periods of growth impairment due to illness, starvation and other unfavourable conditions) is characterised by repetitive series of broadened mini growth spurts.