The most oft-quoted average rate of human hair growth is 6 inches (15 centimeters) per year.
However, the majority of studies measuring the rate of hair growth didn't take into account the race of study participants. It's known, for instance, that Caucasian hair differs from Asian and African hair in several ways, e.g., density (how closely hair strands are packed together) and the angle of hair growth.
A 2005 study in the International Journal of Dermatology also found a difference among races in the rate of hair growth. For example, Asian hair grows the fastest, while African hair grows the slowest.
The average hair growth rate of Asian female participants was nearly 6 inches per year. Comparatively, African female participants' hair grew 4 inches (10 cm) per year, while Caucasian female participants' hair grew a little more than 5 inches (13 cm) per year.
The hair growth rate of the male participants didn't significantly differ from that found for women.
Researchers also think that hair grows quicker in the summer due to dietary changes (eating more fruits and vegetables), as well as shifts in hormones and increased exposure to UV radiation.
But any seasonal increase in hair growth would be so marginal as to be unnoticeable, Elizabeth Cunnane-Phillips, a trichologist (hair and scalp dermatologist) at the Philip Kingsley Clinic in New York, told the Huffington Post.