Hair Growth Rate Calculator

How your hair growth rate is worked out

You measure the same section of hair twice, some weeks apart. The difference between the two lengths is what you kept. Adding back whatever was trimmed in between gives what actually grew from the scalp. Dividing by the time between the checks turns both into a monthly rate.

months            = days between checks ÷ 30.44
retained a month  = (second length − first length) ÷ months
growth a month    = (second length − first length + trimmed) ÷ months
projected a year  = growth a month × 12

What the result is compared with

The popular figure is about half an inch a month. It is a rule of thumb, not a measurement of anyone in particular. A study of hair growth across groups measured Afro-textured hair at about 256 micrometers a day, roughly 0.31 in a month — slower than the popular figure — and later work showed that growth, diameter and curl vary widely within every group. So the verdict uses wide bands: below 0.25 in a month counts as slower, above 0.6 in as faster, and everything in between as about average.

Why short gaps are unreliable

A tape measure on textured hair is easily off by about 0.25 in — the hair is stretched a little more or less, or the tape starts at a slightly different spot. Over four weeks that is half of the growth you are trying to measure. The calculator warns you when your checks are under 8 weeks apart and shows how much the measuring error alone could move the result.

What it cannot tell you

The calculator cannot tell you why your hair grows at the rate it does, or predict how it will change. Growth slows with age and can drop after illness, childbirth or stress. If you are losing more hair than usual or see thinning patches, see a board-certified dermatologist. To turn your rate into dates, use the hair growth calculator; to see how long your hair could get, try the terminal length calculator; and to keep measuring month by month, use the length retention tracker.