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.
Hair's breadth
This measurement is not a precise one. Human hair varies in diameter, ranging anywhere from 17 μm to 181 μm. One nominal value often chosen is 75 μm, but this – like other measures based upon such highly variant natural objects, including the barleycorn – is subject to a fair degree of imprecision.