Some people stop cutting their hair and watch it flow down to their waist. Others give up the scissors for years and find it just won't grow past their shoulders. It's tempting to blame shampoo or split ends. But the real reason is set much deeper, inside each hair follicle.
The short answer: every scalp hair grows for a limited time, usually somewhere between two and seven years, and that period is largely set by your genes. When the growth phase ends, the hair rests and eventually falls out. So the longest your hair can get depends on how fast it grows and how long each strand keeps growing.
The Hair's Hidden Timer
Hair grows in cycles. The active growing stage is called anagen. During this phase, cells at the root divide quickly and push the hair shaft outward. Scalp hair stays in anagen for two to seven years, and that length of time is genetically determined.
Next comes catagen, a short transition of about two to three weeks when the hair stops growing and gets cut off from its blood supply. Then comes telogen, a resting phase of about three months. After that, the old hair is shed and a new one starts the whole cycle again.
At any moment, up to 85 percent of your hair follicles are in the growing phase. Each strand is on its own schedule, which is why you don't suddenly lose all your hair at once.
Doing the Math
Hair is commonly said to grow about one centimeter a month, a little under half an inch. In reality it varies quite a bit. Studies have found scalp hair growing anywhere from about 0.6 to 3.36 centimeters a month. Hair tends to grow more slowly as we age, and thicker hairs generally grow faster than thin ones.
Put those numbers together and the limits make sense. At about a centimeter a month, a hair that grows for two years would reach roughly 9 inches. One that keeps growing for seven years could pass 2.5 feet. That's why two people who never cut their hair can end up with very different lengths.
The same idea explains your eyebrows. Their growth cycle finishes in around four months, compared with three to four years for the scalp. That's why eyebrows never grow long enough to need a ponytail.
The World Record Holder
Then there's Xie Qiuping of China. She began growing her hair in 1973, at age 13. When it was measured on May 8, 2004, it was 18 feet 5.54 inches long, earning her the Guinness World Record for the world's longest hair.
Hair like that is extremely rare. Her strands clearly kept growing for far longer than the usual two to seven years. One estimate of hair lengths in the United States suggests only about 70 people have hair that reaches their ankles.
What Can Cut the Cycle Short
Your genes set the basic timer, but the body can interrupt it. Under extreme stress, as much as 70 percent of hair can slip into the resting phase early and start to fall out. Poor nutrition can also cause hair loss. If you notice sudden shedding, it's a good idea to talk to your doctor.
So if your hair seems to stall at a certain length, it's probably not your fault or your shampoo's. It's simply your follicles following the schedule they were born with. Long or short, it's a pretty impressive bit of biological timekeeping.
