The rates for hair growth are affected by a number of factors including genetics, diet and hormones. Sex hormones cause beard hair in men to grow more quickly; pregnancy hormones in women contribute to both swifter hair growth and thicker hair, by locking hair into the anagen (growing) phase for the duration of the pregnancy. Taking supplements of biotin (Vitamin H) may also contribute to hair growth rates.
How Fast Does Hair Grow On Average?
Hair slows down or speeds up growth depending on various factors, and differs from person to person in any case; so any estimates cannot be used as a prediction for personal growth rates. One oft-cited figure is 0.44mm a day; another is half an inch per month. Clarence R. Robbins in Chemical and Physical Behavior of Human Hair (Springer, 2002) p. 9, states confidently:
“Hair grows at about 16cm/year (~6.2 inches/year) on the vertex or the crown area of the scalp. It grows at a slightly slower rate (~14cm/year) in the temporal area and generally at even slower rates on other body regions (e.g. ~10cm/year) in the beard area”.
Robbins also states that hair growth may slow down gradually due to ageing.
How to Measure Hair Growth
Hair is properly measured from the start of the hairline at the forehead, not from the nape of the neck. The tape measure is brought across the top of the head and down the back, reaching until the tips of the hair. As a result, hair which is 16 inches long will not hang 16 inches down the person's back, and only some of those hairs will actually be 16 inches long.
Wet hair stretches, so for accuracy hair should be measured when dry: or if aiming for consistency rather than strict accuracy, the hair should always be wet or dry each time it is measured. People with curly hair sometimes find it easiest to measure wet hair, as wetting it releases the curl; curly hair should always be stretched to straightness before measuring. This allows curly-haired people to recognise that their hair is in fact growing, despite it not appearing to reach any further down their backs!
'Racer' Hairs and Uneven Hair Growth
People who haven't trimmed their hair for a while often notice a few extra-long 'racer' hairs which grow faster than the others. This is normal. Other variations include hair growing faster on one side of the head, resulting in an uneven hemline, or hair which grows faster in the middle of the head, resulting in a V-shaped hemline.
See also Factors Affecting the Length of Hair Growth.
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