How this calculator works
Scalp hair grows in cycles, and at any moment most follicles are in a growing phase, so the overall length of a head of hair increases at a fairly steady average rate. That rate is commonly given as about 1.25 centimetres, or half an inch, a month, which is 15 centimetres a year. The calculator treats growth as linear over the period you enter. It ignores trims, breakage and the different growth rates on other body sites, all of which can make measured length differ from the estimate.
Formula
Length gained = growth rate x months. Final length = current length + length gained. Months to target = (target length - current length) / growth rate. Days to target = months x 30.4375.
Worked example
At 1.25 cm a month over six months, hair gains 7.5 cm. From a current length of 10 cm, that brings it to 17.5 cm. To reach 20 cm from 10 cm at the same rate takes 8 months, or about 243.5 days before trimming.
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Frequently asked questions
Is 1.25 cm a month correct for everyone?
It is a useful average, not a personal measurement. Growth varies with age, genetics, hormones, nutrition, stress and season, and different parts of the scalp grow at slightly different rates. If you want your own rate, measure from the same point twice, several months apart.
Does the calculator include trims?
No. A trim removes length, so the final length will be shorter than the estimate unless you add the trimmed amount to the target. People who trim regularly often find that visible growth is slower than the raw rate suggests.
Can I use this for beard or body hair?
The arithmetic is the same, but the rate is not. Beard, eyebrow and body hair grow at different rates and have shorter growth cycles. Enter a rate that matches the area you are asking about rather than the scalp average.
Why does the time to target use months of 30.4375 days?
That is the average length of a calendar month over a full year, counting leap years. It converts months into a day count without switching between different month lengths.