Ersatzrate

Ersatzrate, literally meaning replacement rate, is a demographic concept describing the level of fertility required for a population to maintain a stable size in the long term, assuming zero net migration and stable mortality. In practice, it is often approximated by a level of total births per woman that would keep the population constant given current mortality conditions. The value commonly cited for many developed countries is about 2.1 births per woman, though this figure is an approximation that depends on age-specific mortality and the likelihood that children survive to reproductive age.

The replacement rate is linked to the broader framework of fertility and population dynamics. It depends on life expectancy and mortality patterns; higher child mortality raises the replacement rate because more births are needed to ensure enough individuals reach reproductive age, while lower mortality lowers it. It is distinct from the crude total fertility rate at a point in time, which measures births per woman in a cohort without directly accounting for survival or migration. It is also related to, but not identical with, the net reproduction rate, which estimates the expected number of daughters a newborn girl will have over her lifetime under current fertility and mortality conditions.

Migration can alter actual population growth independently of the replacement rate. In contexts with significant net migration, the long-term population trajectory may diverge from what the replacement rate alone would suggest, since migrants can contribute to or diminish the number of people of reproductive age.

See also: total fertility rate, replacement level fertility, net reproduction rate, demographic transition.