On average, the hair grows to about a 1/2 inch per month. This means that the hair can be grown to approximately 6 inches in a year.
There are several factors that also affect the speed of hair growth and these include age, sex and ethnicity. As people grow old, the speed at which the hair grows slows down to as little as 0.1 inch per month.
In a study published in the International Journal for Dermatology in 2005, hair growth in women also differs according to their ethnicity. The study shows that Asian women's hair grows the fastest compared to other races. The average speed of hair growth among Asian women is nearly 6 inches per year.
In comparison, the hair on women with African ethnicity grows only an average of 4 inches in a year and the hair on Caucasian women grows to slightly more than 5 inches in the same length of time. The speed of hair growth on men, on the other hand, does not differ as much as women.
The hair grows due to cell division. As new cells are created at the root, it pushes the hair forward out of the follicle, making it longer. The average human head has approximately 100,000 strands of hair at any given time. Each hair can stay on the head between 1 to 6 years before falling out and a new one is created from the root.