Human hair grows on average 1.25 centimetres (0.5 inches) per month, which is equivalent to about 15 centimetres (six inches) per year. There are several variables that can affect hair’s growth rate such as age, health and genetics. Each hair grows in three stages, the first being the anagen phase where most growth occurs. The longer your hair remains in this stage dictates how long and quickly it develops; this can last between two and eight years and is followed by the catagen (transitional) and telogen (resting) phases. Hair growth rates vary across different areas of the head, with that on the crown growing the fastest.
Answered by Laura Mead, Science Museum.