Subtracting a past population value from the present population value, and dividing this result by the past population value, yields the population-growth rate. Multiplying the population-growth rate by 100 yields the growth percentage.
Population growth is a change in population size during a particular time period. In the simplest calculation, growth rate is equal to [P(t2) - P(t1)]/P(t1), where P(t1) is a past population value and P(t2) is the present population. This calculation yields a growth rate for any chosen time range. It is common to express growth rate as a percentage by multiplying the rate by 100. Dividing the percent population growth by the number of years between t1 and t2 gives an annual growth percentage. For example, a population that experiences 10 percent growth in 10 years has a 1 percent annual growth rate.
The population growth calculation is also an excellent tool for predicting future populations, given a known growth rate. In this case P(t2) is a future population, and the unknown variable, and P(t1) is the present population.
Another method of predicting a future population is to find doubling time. Doubling time is the result of dividing 70 by the percent growth. In this calculation 70 is a constant that derives from the natural log of 2.