Fertility: In order to study at which speed, at which the population is increasing, fertility rates are used which are of various types? Important among these are:
Crude birth rate: it is the simplest method of measuring fertility. It acts as an index of the relative speed at which additions are being made to the population through child birth. In this method the number of births is related to the total population. Since it is only a live birth that signifies an addition to the existing population, live births alone are considered in measuring fertility, thus excluding still births.

The annual crude birth rate is defined as:
Crude birth rate = annual births/annual mean population × 1000
In this measure the births are related to the mean population and not to the population at a particular date. The crude birth rate of a given year tells us that what rate births have augmented the population over the course of the year. 
The crude birth rate usually lies between 10 and 55 per 1000. The level of the crude birth rate is determined by:
(i) The sex and age of distribution of the population and
(ii) The fertility of the population i.e. the average rate of child bearing of females.
A relative high crude birth rate can be recorded if the sex and age distribution is favorable even though fertility is low, i..e. countries with a relatively large proportion of population in the 15-50 years group will have a relatively high crude birth rate, other being equal.
Specific fertility rate: the concept of specific fertility arises out of the fact that fertility is affected by a number of factors such as age, marriage, state or region, urban rural characteristics, etc. when fertility rate is calculated on the basis of age distribution; it is called the age specific fertility rate. While calculating age specific fertility rate women of different ages in the child bearing age are placed in small age groups so as to put then at par with others of child bearing capacity. The fertility of women differs from age to age and, therefore, the grouping of women of different ages is essential. The capacity to bear children is much higher in the age group 20 to 25 than in the age group 40 to 45.
S. F. R = (Number of live births which occurred to females of a specified age group of the population of a given geographic area during a given year/mid year female population of the specified age-group in the given geographic area during the same year) × 1000
General fertility rate: this rate refers to the proportion of the number of children born per 1000 of females, the reproductive of child bearing age, this numerator of this rate would remain the same as the crude rate, but the denominator would be limited to the sex group of the population able to contribute to the birth rate. The formula for such a rate is
General fertility rate (G. F. R) = (number of live births which occurred among the population of a given geographical area during a given year/mid year female population of ages 15 to 49 in the given geographical area during the same year) × 1000
The computation of F.G.R requires that a decision be taken beforehand as to which years of the life a woman should be included in the child bearing period. Although the practice varies in this respect, generally the child bearing age is taken 15 to 50 years. Births to mothers under 15 and above 50 are so rare that they are not recorded separately but are included in the age group 15 and 49 respectively.  

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