The slow, steady growth on your head
Hair is one of the fastest-growing tissues in the body. Each strand on your scalp grows about a centimetre and a quarter every month, roughly the width of a fingernail, and with around 100,000 of them working at once your head produces many metres of new hair every single day. Most of it gets trimmed off or falls out long before you notice, but if a single hair could keep growing from the day you were born and never be cut, it would by now be taller than most buildings you walk past.
The picture above shows that imaginary strand next to familiar tall things, and it grows as you change the date or the growth rate. In reality no single hair lasts that long: each one grows for a few years, rests, and falls out to make way for a new one.
How we calculate it
We multiply the months since your birth by your chosen growth rate, 1.25 cm a month by default, which gives the length of one hair that never stopped. For the whole head we multiply that by 100,000 hairs, a typical number for an adult scalp. Babies are born with very fine hair and grow more over the first years, so the early part of the total is a little generous; it evens out over a lifetime.
| Time | Growth at 1.25 cm a month |
|---|---|
| One day | about 0.4 mm |
| One month | 1.25 cm |
| One year | 15 cm |
| Ten years | 1.5 m |
| Forty years | 6 m |
Worked example. Someone aged 36 years and 6 months has lived about 438 months. At 1.25 cm a month, one uncut strand would be 547 cm, or 5.5 metres, about the height of a giraffe. Their whole head, at 100,000 hairs, has grown roughly 547 km of hair, enough to stretch from London to Edinburgh as the crow flies.
Why haircuts never seem to end
At 15 cm a year, a short haircut grows out within a couple of months, which is why barbers see regulars every four to six weeks. Many people donate long hair to charities that make wigs; most ask for at least 20 to 30 cm, which is about 18 months to two years of growth.
Questions people ask
How fast does hair grow?
Scalp hair grows about 1.25 cm (half an inch) a month on average, or roughly 15 cm a year, according to the American Academy of Dermatology. Speed varies with age, genetics and health.
Why doesn’t uncut hair grow forever?
Each hair grows for a set period, usually two to seven years, then rests and falls out so a new one can start. That growth window limits how long your hair can get, which is why most people’s uncut hair stops somewhere between shoulder and waist length.
How many hairs are on a head?
Typically 80,000 to 120,000, depending partly on hair colour. Blondes tend to have the most strands and redheads the fewest. We use 100,000.
How much hair do we lose a day?
Losing 50 to 100 hairs a day is normal, as old hairs finish their cycle and drop out.
Sources. American Academy of Dermatology (hair growth rate and daily shedding); hair count ranges from dermatology references. Calculations run in your browser; nothing you enter is sent anywhere.