The answer is in a timer your genes set before you were born.
“My hair just won’t grow.” It isn’t quite true. Your hair is growing right now, at about a centimetre a month. What differs from person to person is how long each hair is allowed to keep going.
Three stages, one cycle
Every hair on your head follows the same cycle.
Anagen is the growing phase. The follicle is working flat out, pushing out new hair. It lasts years.
Catagen is a short transition of a few weeks. Growth stops and the follicle shrinks back.
Telogen is the rest. The hair sits there for around three months, then falls out, and a new one starts underneath.
At any moment, roughly 85 to 90% of your scalp hairs are growing and the rest are resting or shedding. That’s why you lose a fair few hairs every day (50 to 100 is normal) without ever going bald. Each follicle runs on its own clock, so they don’t all fall out at once.
The 2 to 7 year timer
On the scalp, anagen lasts anywhere from about two to seven years. That’s the number that matters.
The sum is simple. A hair grows about 12 to 15 centimetres a year. Two years of growth gets you roughly shoulder length. Seven years gets you towards a metre. Your hair’s maximum length is its growth rate multiplied by the length of your growth phase. When the timer runs out, the hair falls out. It doesn’t matter how well you treat it or how many vitamins you take.
So two people can grow their hair at the same speed, with the same care, and one ends up with hair to her waist while the other’s stalls at her collarbones. Their hair isn’t growing differently. One has a seven-year timer and the other a two-year one.
What sets the timer
Mostly genes. One of the best-studied is FGF5, which makes a signal that tells a follicle to leave the growth phase. A strong signal means a short anagen and shorter hair. A weak one lets the hair keep growing. Researchers reported in 2014 that FGF5 is a crucial regulator of hair length in humans. The same idea shows up in animals: the long-haired Angora breeds of rabbits, cats and goats carry changes in the same gene.
Eyelashes show the timer clearly. They run on exactly the same cycle, but their growth phase lasts only weeks, so they stop at about a centimetre. Eyebrows and body hair are the same. We lost most of our body fur and kept the crown, and part of the story is that evolution left the scalp’s timer long and shortened the rest.
The world record shows what a long timer can do. Xie Qiuping’s hair was measured at 5.6 metres in 2004. At a normal growth rate that would take decades, and her follicles simply never got the message to stop at seven years.
Other things that move the number
Age. The growth phase tends to shorten as we get older, so hair gets thinner and stops at shorter lengths.
Hormones. In pregnancy, more hairs stay in the growth phase. After the birth many shed together, which is why new mothers often lose a lot at once.
Stress and illness. A shock can push many follicles into the resting phase at once. The shedding shows up two or three months later.
Breakage. This matters more than people think. The ends of long hair are old. Hair that’s 60 centimetres long is around four years old, and it has been brushed, heated, dyed and tied for that whole time. If it snaps off as fast as it grows, you’ll never gain length, whatever the timer says. Tightly coiled hair is more fragile at each bend, and it shrinks when dry, so a 30-centimetre strand can look a third of that. A lot of “my hair won’t grow” is really “my hair won’t keep its length”.
Averages differ between populations in some studies, but the spread within any group is far larger than the gap between them. Your timer is your own.
So what can you do?
You can’t lengthen your anagen. You can keep what you grow: cut less often only if the ends are healthy, go gently with heat and tight styles, and protect hair from friction. Short hair is no failure. It might just be a short timer.
Hair seems like the most personal thing about us, and it turns out to run on a clock we never set.