Hair Growth Calculator

Hair growth time is how long new growth at your scalp takes to add the length you want. Scalp hair grows at a fairly steady speed, so most of the answer is simple division. What changes the answer is your own growth speed and how much each trim takes back.

What Hair Growth Rate Measures

Your growth rate is how much length a hair adds each month while it is growing. Scalp hair grows about 0.35 mm a day, or roughly 1 cm (0.4 inch) a month¹. The rate measures speed only. It does not tell you how thick your hair is or how long each hair can keep growing before it sheds.

The Formula

Months to goal = (target length - current length) / growth rate per month

With trims: growth between trims = monthly rate x months between trims, minus the length cut

The 1 cm a month figure comes from hair physiology reviews¹. A study that photographed the same patch of scalp a few days apart found group averages of about 0.8 and 1.2 cm a month².

How This Calculator Computes Your Result

Enter your current and target length in cm or inches, measured from the scalp to the ends. Pick a rate: Slower (0.8 cm or 0.3 inch a month), Average (1.0 cm or 0.4 inch, the default), Faster (1.2 cm or 0.5 inch), or your own. If you get trims, add how much is cut and how many weeks apart. The calculator counts forward day by day (an average month is 30.44 days), takes off each trim on its date, and stops trimming at your goal. You get the months needed, a goal date, a timeline, and a slower to faster range.

What Your Result Means

Months needed to add length, without trims:

Treat the main result as the middle case. With the cm presets, the slower rate takes 50% longer than the faster rate (about two thirds longer with the rounded inch presets).

Trims matter more than most people expect. Growing from 10 to 20 inches at 0.4 inch a month takes 25 months with no trims, but about 44 months with a half inch trim every 12 weeks.

The result also shows how old the hair at your ends will be at your goal. Past about 4 years, you will see a note, because many hairs shed before then.

How to Measure Correctly

• Pick one section, such as the middle of the back of your head, and measure from the scalp to the ends with a soft tape.

• Measure your goal the same way, from the same spot.

• Gently stretch curly or coily hair straight before measuring, or shrinkage will hide length.

• To find your own rate, measure the same section once a month for 2 to 3 months and enter the average monthly gain as a custom rate.

The Hair Growth Cycle and Your Maximum Length

Each follicle cycles through three phases. Anagen is the growing phase and can last several years¹. Catagen is a short shift of a few weeks, and telogen is a resting phase that ends when the hair sheds¹. A typical scalp follicle grows hair for almost 4 years, then rests for about 4 months, and about 85% of scalp hairs are growing at any one time³.

This is why hair has a natural length limit. At 1 cm a month, 4 years of growth is about 48 cm (19 inches). People with longer growing phases can grow much longer hair. Shedding is part of this cycle, and losing 50 to 100 hairs a day is normal⁶.

Why Some Hair Grows Faster

In a study of 38 young adults born in Africa, hair grew a mean of 256 micrometers a day (about 0.8 cm a month), compared with 396 micrometers a day (about 1.2 cm a month) in a Caucasian comparison group². Growth did not differ between men and women or between areas of the scalp, and people varied widely within the group, from 150 to 363 micrometers a day².

Age mainly changes the cycle. A study that followed 10 men for 8 to 14 years found that, with age, the growing phase gets shorter and hairs get thinner⁴. A shorter growing phase means a lower maximum length. Body site matters too: the resting phase lasts a few weeks for eyelashes and up to nearly a year for scalp hair¹. This calculator is for scalp hair only.

Some popular ideas do not speed growth. Hair is made in the follicle under the skin¹, so cutting the ends does not change its speed. A dermatology review found that biotin does not need to be supplemented in healthy people and that taking it for hair is not supported by large studies⁵. Too much vitamin A can even cause hair loss⁵. Fixing a true deficiency may help, and a doctor can test for that⁵.

When Shedding or Hair Loss Needs a Doctor

Heavy shedding often follows a stressor such as giving birth, a high fever or surgery⁶. It usually starts about 3 months after the trigger³, and fullness tends to return within 6 to 9 months⁶. The American Academy of Dermatology lists signs worth a closer look: a widening part, a thinner ponytail, a receding hairline, sudden bald patches, clumps falling out when you brush, or an itchy, burning or scaly scalp where hair is lost⁷. A dermatologist can tell whether you have hair loss, extra shedding, or both⁶.