Analysts base population growth estimates on past data and future expectations. If you know how to use these projections, you can calculate the population projections for different areas years into the future. Population projections are made for a variety of important purposes. For example, a city may calculate population projections to determine future needs for services, such as police, fire and schools. A country might use population projections to determine the future expected costs for welfare programs or retirement benefits. To calculate population projections, you need to know the estimated growth or decrease rate, the current population and how far into the future you want to predict.

Divided the projected growth rate by 100 to convert to a decimal. For example, if the population of a city is projected to grow at a rate of 1.8 percent per year, divide 1.8 by 100 to get 0.018.

Add 1 to the projected growth rate expressed as a decimal. In this example, add 1 to 0.018 to get 1.018.

Raise the result to the number of periods that will elapse before the projected time. For this example, if you want to project the population of the city 10 years from now, raise 1.018 to the 10th power to get 1.195302368 because the projected growth rate is per year.

Multiply the result by the original population to find the projected population. In this example, if the city started with a population of 840,000, multiply 1.195302368 by 840,000 to get 1,004,054, meaning in 10 years the population is projected to grow to just over 1 million people.