The crude birth rate depends on how many people of childbearing age a country has. Demographers want measures that compare countries fairly and say something about families and health. Three are used everywhere.
Key idea
The total fertility rate (TFR) is the average number of children a woman would have in her lifetime at today's rates. Replacement level is about 2.1 in countries with low death rates. The infant mortality rate (IMR) is deaths of babies under one year per 1,000 live births. Life expectancy is the average number of years a newborn would live at today's death rates.
Why it works
Two parents need to be replaced by two children who grow up to have children of their own. It takes slightly more than 2.0, because a few children die before adulthood and slightly more boys than girls are born (about 105 boys per 100 girls). Where more children die young, replacement level is higher than 2.1. The infant mortality rate divides by births, not by the whole population, so it measures how safe the first year of life is.
- World TFR: about 2.3 children per woman (UN, 2024); more than half of all countries and areas are below 2.1.
- World life expectancy at birth: about 73.3 years (UN, 2024).
Worked examples
Worked example: Infant mortality
A region had 50,000 live births in a year, and 1,200 babies died before their first birthday. Find the IMR.
- IMR = infant deaths ÷ live births × 1,000.
- 1,200 ÷ 50,000 = 0.024.
- 0.024 × 1,000 = 24.
Answer: 24 infant deaths per 1,000 live births.
Worked example: Above or below replacement?
Country A has a TFR of 1.4; Country B has 3.8. What does each suggest, ignoring migration?
- A is below 2.1: each generation is smaller than its parents' generation, so in the long run the population shrinks.
- B is well above 2.1: each generation is larger, so the population grows.
- Neither changes overnight: an age structure takes decades to catch up.
Answer: A: long-run decline; B: growth.
Common mistake
Students think a TFR of 2.0 keeps a population steady. Two children per woman falls just short, because not every child survives to have children and more boys than girls are born. Replacement is about 2.1 in rich countries and higher where child deaths are common. Also, a low TFR does not mean a population shrinks at once; that depends on its age structure.