Total fertility rate versus general fertility rate

Key differences

Difference type For TFR For GFR
essential purpose describing how many children a female would have if at each age she behaved like the average of the women currently at that age describing how many children have been added in a particular year, relative to the number of women
relation with age-specific fertility rates it is a total it is a weighted average
relation with age-specific fertility rates, weighting used we use a weighting of 1, i.e., we just add up the ASFR values we weight by the population of women of that age
scaling used to express the value scaled to a denominator of 1, i.e., we report the TFR per woman scaled to a denominator of 1000, i.e., we report the GFR per 1000 women. This is simply because the GFR value per woman would be too small, and we generally prefer to use a scaling where we get reasonably large and easily comparable numeric values.

Cases where they convey equivalent information

TFR and GFR would be easily related in the following two cases:

• All the age-specific fertility rates are the same. In this case, the GFR would equal all the equal age-specific fertility rate values, and the TFR would be the product of that value and the number of childbearing years.
• The population size is the same for each age. In this case, the GFR would equal the average of the age-specific fertility rate values, and the TFR would be the sum of the age-specific fertility rate values. In this case again, the TFR would equal the product of the GFR and the number of childbearing years.

Of course, neither case occurs in practice: there is considerable variation in the age structure of the female population (see the population pyramid for more), and considerable variation in the age-specific fertility rates.