KeraHow fast hair actually grows, and what can change it
Scalp hair grows roughly 0.3 to 0.4 mm a day. Nothing on the market meaningfully speeds that up — but plenty of things stop you keeping what grows.
The short answer
Scalp hair grows about 0.3 to 0.4 mm per day, or roughly 1 to 1.5 cm a month and 12 to 15 cm a year, with modest variation by genetics, age and ethnicity. No product reliably increases that rate. Hair that appears stuck at a length is either at its terminal length — determined by how long your growth phase lasts — or breaking at the ends as fast as it grows, which is a different problem with a real solution.
Ask what makes hair grow faster and the internet will offer you biotin, rosemary oil, inversion methods, rice water and a scalp brush. The honest answer is duller and more useful: hair grows at a rate you were largely born with, and the variable you control is not growth. It is how much of it you keep.
How fast does hair grow? The number
Scalp hair grows at approximately 0.3 to 0.4 mm per day. That is:
- Around 1 to 1.5 cm a month
- Around 12 to 15 cm a year
The variation between individuals is real but modest. Growth rate declines gradually with age. There are consistent average differences between hair types, with Asian hair tending to grow somewhat faster and tightly coiled hair somewhat slower on average — though the coiled-hair figure is confounded by the fact that a coiled fibre's straightened length is much greater than its visible length, so growth is systematically under-perceived.
Growth is also slightly faster in warm months, which is small enough that you will never notice it on your own head.
Notably, hair on different parts of the body grows at different rates and has vastly different cycle lengths — eyebrow hair has an anagen phase measured in weeks, which is why it never reaches scalp length no matter what.
Terminal length: why your hair stops
The single most useful concept here, and the one that answers "why can't my hair get past my shoulders".
Each follicle's anagen growing phase has a genetically determined duration, typically two to seven years. At the end of it, the hair is released regardless of length. Multiply your growth rate by your anagen duration and you get your terminal length:
- A 2-year anagen at 1.25 cm a month gives roughly 30 cm — around shoulder length.
- A 4-year anagen gives roughly 60 cm — mid-back.
- A 6-year anagen gives roughly 90 cm — waist.
If your hair reaches a certain length and stops, and the ends look healthy, you are probably at your terminal length. Nothing changes it. This is a genuine constraint and it is worth knowing so you stop spending money on it.
The important caveat: most people who believe they have hit terminal length have not. They are losing length at the ends as fast as they gain it at the roots, and the giveaway is the state of those ends.
Retention: the variable you actually control
Here is the arithmetic that reframes everything.
You grow 15 cm a year. If you lose 15 cm a year to trims and breakage, your hair is exactly the same length in December as it was in January — and it will be the same next December. People in this situation conclude their hair does not grow. It grows fine. It is not being kept.
Reduce breakage so you only need 5 cm of trimming a year, and you gain 10 cm. That is the difference between "my hair never grows" and shoulder-to-mid-back in two years, achieved without changing a single thing about the follicle.
So the strategy for length is not growth. It is:
Stop breakage at the source. Detangling from the ends up with slip, not from the roots down through knots. This is the highest-value habit in hair care and it is free.
Cut heat exposure. The ends of long hair have been ironed hundreds of times. Lower the temperature and the ends survive longer.
Protect overnight. Eight hours of friction against cotton, every night, for years, is a large fraction of total mechanical damage.
Trim splits before they travel. A split left alone extends up the shaft, so you end up cutting more later. Dusting rather than blunt-trimming keeps the length while removing the damage.
Protect from sun, chlorine and salt. All three degrade the fibre, and all three are avoidable with a hat or a rinse.
What genuinely affects the rate
Very little, and all of it is downward.
Nutritional deficiency. Insufficient protein, iron, zinc or overall calories slows growth and pushes hairs into shedding. Correcting a real deficiency restores normal growth — it does not exceed it.
Illness and medication. Chemotherapy is the extreme case; many other drugs and conditions slow or interrupt growth.
Age. Gradual decline in rate, and shortening anagen phases, for essentially everyone.
Hormones. Thyroid dysfunction in either direction affects hair growth, and androgenetic changes shorten anagen in susceptible follicles.
The one intervention with real evidence for changing the outcome is minoxidil, which extends the anagen phase — and it is a medication for hair loss, not a growth accelerator for people with normal hair.
Setting an expectation you can actually hold
At 1.25 cm a month, six months is 7.5 cm — enough to see in a photograph, not enough to notice in a mirror day to day.
Which is why measuring matters. Take a photo in the same position and the same light every month, or measure from a fixed point on your body rather than judging by feel. The technique takes two minutes to learn, and it is the difference between knowing your routine is working and guessing.
Common questions
Does trimming make hair grow faster?
No. The follicle is under the skin and has no information about the ends. Trimming helps you keep length by removing splits before they travel upward.
Do supplements speed growth?
Correcting a genuine deficiency can restore normal growth. Taking supplements without a deficiency does not accelerate it beyond normal, and some — excess vitamin A, excess selenium — cause shedding.
Why does my friend's hair grow faster?
Partly genetics in growth rate, but mostly a longer anagen phase and less breakage. Growth rates vary less between people than retention does.
How long before I notice hair growth?
About three months for a change to be visible in a photograph, and closer to six for it to be obvious in a mirror. At roughly a centimetre a month, four weeks of growth is under half an inch — which is why routines judged at four weeks are judged on nothing.
How much does hair grow in a week?
Roughly 2 to 3 mm, or about a tenth of an inch. That is the number behind every realistic hair goal, and it does not change with shampoo.
Does hair grow in cycles?
Every follicle does, independently: years of active growth, a brief shutdown, then about three months of rest before the hair is released. Because the cycles are out of step with each other, we shed continuously rather than moulting.
Kera offers hair-care lifestyle suggestions, not medical advice. It does not diagnose or treat any condition.