How hair actually grows

Each follicle on your scalp moves through a repeating cycle, and crucially, each one does so on its own schedule. Your hundred thousand or so follicles are not synchronised, which is why you do not moult.

Anagen: the growing phase

The active phase, when cells at the base of the follicle divide and push the strand upward. This lasts somewhere between two and seven years for scalp hair. Because hair grows at roughly a centimetre a month, the length of your anagen phase is what sets your maximum hair length. Someone with a three-year anagen will struggle to grow hair past a certain point no matter what they do, and someone with a seven-year anagen can reach their waist without trying.

Catagen: the transition

A short phase of two or three weeks. The follicle detaches from its blood supply and begins to shrink. Growth stops. Only a small percentage of your hair is in this phase at any moment.

Telogen: the resting phase

Around three months. The strand sits in place, no longer growing, until a new hair beginning its anagen phase pushes it out. That shed strand is the one you find in your comb.

At any given moment roughly 85 to 90 percent of your scalp hair is in anagen and around 10 percent is in telogen. Since telogen hairs are constantly reaching the end of their rest, you lose hair every day. The commonly quoted figure is 50 to 100 strands, and it is a reasonable rule of thumb rather than a precise threshold.

This is not hair loss. It is the cycle working. A shed telogen hair is being replaced by a new anagen hair from the same follicle. Hair loss, properly speaking, means either follicles producing progressively thinner strands or follicles stopping altogether.

About one to one and a half centimetres a month, which is roughly half an inch. Over a year that is around 12 to 15 centimetres, and only if none of it breaks off at the ends in the meantime.

That rate is largely genetic. It varies slightly with age, health and season, but not by much, and nothing applied to the outside of your head meaningfully accelerates it. This is the single most useful fact to hold on to when reading marketing copy.

Because the cycle runs in years and months rather than days, any genuine change in your hair takes a long time to become visible. A follicle that begins producing a healthier strand today has to grow it out before you can see it.

Practically, this means three things. Judge nothing in under three months. Expect visible length change to take a year. And be immediately suspicious of anything claiming results in weeks, because the biology does not permit it.

It also means that protecting the hair you already have matters at least as much as anything you do at the root. Hair that grows well but snaps off at the ends never gets longer. Our guide on shedding and breakage covers how to tell which is happening.

• Genetics, which set your anagen length, growth rate and strand diameter, and account for most of the variation between people.
• Age. Anagen phases tend to shorten over a lifetime and strands often become finer.
• Hormonal change, including pregnancy, postpartum, thyroid conditions and menopause.
• Serious nutritional deficiency, particularly iron and protein. Note that correcting a deficiency helps; exceeding your requirements does not add a bonus.
• Significant illness, surgery, rapid weight loss or acute stress, any of which can push many follicles into telogen at once.
• Some medications. Worth asking a doctor rather than guessing.

Notice what is absent from that list: almost everything sold as a growth product. Scalp care and gentle handling are worth doing because they let hair reach its natural potential and reduce avoidable damage. That is a real benefit, and it is different from making hair grow faster.