How are births deaths and net migration used to estimate a population after a census?

To estimate a population between censuses, demographers use the fundamental components of population change to calculate the shift from a known starting point.

For an open population—one where people move in and out—the population at the end of a specific interval is determined by the following equation:

Population (end) = Population (start) + Births – Deaths + In-migrants – Out-migrants1

In this context, births and deaths represent vital statistics, while the net difference between in-migration and out-migration accounts for migration patterns.12

While a census provides a baseline, it is rarely perfectly accurate, and the reliability of these estimates depends heavily on the quality of local registries for births and deaths. Because demographic trends can shift unexpectedly, demographers frequently update these calculations to incorporate newly developed data, ensuring that projections remain as accurate as possible.23