How fast does hair growth?

The rate of hair growth varies considerably but on an average it is one centimetre per month. The growth is greater in summer than in winter. It is maximum in the ages between 15 and 30 years. Cutting or shaving does not make hair grow faster. Neither does trimming affect the growth rate in any way (though trimming does make your hair look good   by eliminating the straggly ends)

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Each hair undergoes cyclic activity in three phases : the growth phase or anagen lasts for 3-4 years ; following anagen is a short period of transition or catagen, following which is the phase of telogen or resting . During the resting phase (2-3 month)the hair becomes detached from its root and falls out, as a new growing hair replaces it in the follicle

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At a given time, 10-15% of the scalp hair are in an inactive phase ,ready to fall out, while the remaining are in various stages of growth. A loss of up to 100 hairs per day is normal and is something you should not worry about as most of the lost hair would be replaced. With age , however, the rate of replacement slows down; so as you grow older, there is a natural tendency of some amount of sparseness of your scalp hair.

To know more about Hair and Scalp-Hair and Scalp – What is the structure of hair?

What controls the growth of hair?

1. Family background and racial factors are very important for the quality as well as the quantity of hair. Good Quality long hair runs in families, and so does the all too familiar male types of baldness.
2. Nutrition : Hair, like any other aspect of your looks, is best improved by treating your system well. A good   balanced diet is what is really required. An inadequate diet disturbs the structure ,the growth and the colour of the hair. When there is a total deficiency of protein, the hair becomes sparse, fine, brittle, dry, and light coloured. Then severe hair loss seen after crash diets is primarily due to protein deficiency. High protein diets, especially those containing gelatine and cysteine, increase the rate of hair growth. Deficiency of pantothenic acid leads to premature greying of hair in animals. Lack of essential fatty acids also causes hair to fall .Iron deficiency, especially in women, has been associated with hair loss, while iron supplementation hastens the growth of hair .Zinc deficiency also leads to hair fall .So an ideal diet for hair growth should contain adequate amounts of iron, zinc, protein and fatty acids.
3. Age :With increasing age ,there is a decrease in hair growth. The replacement of shed hair is also incomplete and this results in sparsity of hair. In men, there is a recession of hairline and sometimes definite areas of baldness appear. This baldness is dependent on the male sex hormones-the androgens. Rarely this hormone-mediated baldness is seen in women too.
4. Hormones : A number of other hormonal disorders can cause hair loss -thyroid disorders being the commonest. Though during pregnancy, hair loss slows down or may even stop altogether, excessive hair fall commonly occurs three to four months after delivery. Contraceptive pills also cause excessive hair loss and this stops when the pill is discontinued.
5. Illness : Any severe illness, be it physical illness(surgery, accident, fever, etc.) or even mental stress, can result in hair losses reversible and recovers in about 3-6 months.
6. Drugs :  Many drugs like anticancer medication can give rise to hair fall.