Fertility rate (total fertility rate, TFR)

The total fertility rate (TFR) is the average number of children a woman would have over her lifetime if she experienced the age-specific birth rates observed in a given year throughout her childbearing years. It is a hypothetical, period-based measure rather than a count of children any real cohort of women actually had, since it assumes today's birth rates hold constant for a woman's entire reproductive life.

TFR = sum of age-specific fertility rates across childbearing ages, typically summed across five-year age bands. This differs from the crude birth rate, simply births per 1,000 population in a year: the crude rate is affected by a population's age structure, since more women of childbearing age raises it even with unchanged fertility behavior, while TFR standardizes for age structure and so is comparable across populations and over time.

A TFR of roughly 2.1 is commonly cited as the replacement level in low-mortality populations, though the exact threshold depends on mortality rates and varies by country. Fertility rate shapes the base of the population pyramid and, with life expectancy and migration flows, feeds into the dependency ratio and population projections. A common pitfall is treating a single year's TFR as a firm prediction of completed family size, when actual cohort fertility can differ once women's full childbearing years play out, particularly when women are delaying rather than forgoing childbearing.