Human hair typically grows at a rate of around half an inch (1.25 centimeters) per month, and this rate of growth is consistent across the head. Several factors can influence both the rate and quality of hair growth, including diet, age, and general health. As a general rule, products which claim to increase the rate of hair growth are not effective, although such products can encourage the development of stronger, healthier hair.

When human hair grows, it goes through three states. The first state is the anagen phase, during which hair is actively growing, and it can last from two to six years. During this phase, cells in the hair follicle are actively dividing, pushing the hair out from the head. During the catagen phase, the rate of growth stops as the follicle grows dormant, and in the telogen phase, the hair falls out to make way for new hair growing in.

As human hair grows, nutrients can help to make it stronger and sleeker. Things like gelatin, keratin, vitamin E, and various mineral supplements can improve the overall health of hair, making it glossier, thicker, and less prone to splitting. Many people also use regular conditioners and hot oil treatments to keep their hair in good shape, especially when hair is long. However, nothing can actually speed up the rate of growth, because the growth rate is limited by cell division.

Some factors which can influence hair growth include age and medical conditions; people with thyroid problems, for example, often develop problems with their hair. Hormones associated with pregnancy can actually speed the rate of hair growth, and an assortment of other conditions may influence the condition and growth rate of human hair. In fact, hair health can sometimes be used as an indicator for general physical health.

As humans who have attempted to grow their hair out may have noticed, many people seem to have some sort of hair “set point,” beyond which their hair will not grow. After a certain point, human hair gets too old, and it becomes brittle, split, and unsightly. Some individuals are able to grow their hair out to immense lengths, especially when supplementing with vitamins, while others may find that their hair refuses to thrive much past their shoulders; in this situation, the best thing one can do is find a flattering cut for shorter hair, rather than enduring the frustration of cultivating long hair.