A demographic equation is a formula that measures population change. It estimates the increase or decrease in population by adding the number of births plus the net migration to the previous count, then subtracting the number of deaths.
Demographic equations are used by businesses as well as political entities to estimate the natural increase of a specific population over a given period of time. They do not provide highly accurate results, but rather useful approximations. The net migration is determined by subtracting out-migration from in-migration of the population during that time period. The result is added to the previous count, plus births minus deaths, to give the new value.
Associated principles includes doubling time, which is the time required for a population to double assuming a constant growth rate. This is most accurately calculated by dividing the natural logarithm of two by the exponent of growth, but it can be more simply approximated by dividing 70 by the percentage of growth.
Another associated principle is zero population growth, which occurs when a population does not increase or decrease from year to year. This is the desired state of some political activists who feel that the current global population growth rate is unsustainable over the long term.