Human hair growth
The three stages of hair growth are the anagen, catagen, and telogen phases. Each strand of hair on the human body is at its own stage of development. Once the cycle is complete, it restarts and a new strand of hair begins to form. The rate or speed of hair growth is about 1.25 centimetres or 0.5 inches per month, or about 15 centimetres or 6 inches per year.
Long hair
1972, it was estimated that 24% of American women wore shoulder length hair or longer (44% of women in ages 14 to 44), meaning slightly more than 12% of men and women altogether. Similar frequency was found in 2001, when it was estimated that about 13% of the US adult population, male as well as females, has hair shoulder-length or longer, about 2.4% have hair reaching to the bottom of the shoulder blades or longer, about 0.3% have hair waist length or longer, and only about 0.017% have hair buttocks-length or longer. By extrapolating the above data and the number of hair length records, the number of people with shoulder-length hair or longer in the USA is estimated to be 27 million, those with waist-length hair roughly 900,000, those with buttocks-length hair to be 40,000, with knee length hair to 2,000 and with ankle length hair to 70.