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.
Ponytail
The numerical values used in this equation were the average density of human hair (1.3 grams per cubic centimeter), a bending modulus ( "A" ) of 8 × 10 Newton meter, a linear mass density of hair ( "λ" ) of 65 micrograms per centimeter and the major diameter of the hair (human hair is elliptical in cross section) of 79 ± 16 micrometers. "N" was taken to be 100,000 - the average numbers of hairs on a human head. The constant "E" was that of nylon which is similar to hair in its bend and twist moduli. The length "L" was taken to be 25 centimeters. These values give a length at which gravity bends a hair of "l" = 5.