AP® Environmental Science · Unit 3 · Topic 3.5
Calculating Population Growth Rates
Population growth rate is natural increase (births minus deaths) plus net migration (immigrants minus emigrants), expressed as a percent. That single number tells you whether a population is growing, shrinking, or holding steady.
A country can have more births than deaths and still shrink, because births and deaths are only half the story. People also move. If a country has a healthy natural increase but loses huge numbers of people to emigration, the population can fall overall. Growth rate only makes sense once you count everyone coming and going, not just everyone born and dying.
The full calculation adds two pieces. Natural increase is births minus deaths, per 1000 people. Net migration is immigrants minus emigrants, per 1000 people. Add those two numbers together and you get total change per 1000 people per year. Divide by 10 to turn that into a percent, since 1000 people is the base and percent means per 100. A positive result means growth, a negative result means decline.
Once you have a growth rate, you can project population forward in time: multiply the current population by 1 plus the growth rate as a decimal to get next year's total, and repeat year after year for longer forecasts. This connects growth rate directly to the logistic growth curve and carrying capacity: a population keeps growing as long as its growth rate stays positive, but as resources tighten, births drop, deaths rise, or both, pulling the rate toward zero. It also connects to age structure diagrams, since a country's mix of young and old people predicts whether its growth rate will rise or fall in the future, well before the population itself changes direction.
Key terms
- Natural increase
- The difference between birth rate and death rate, usually expressed per 1000 people.
- Net migration
- The difference between the number of immigrants and emigrants, usually expressed per 1000 people.
- Growth rate
- The overall percent change in population size per year, combining natural increase and net migration.
- Exponential growth
- Population increase that compounds each year, producing a J-shaped curve when the growth rate stays constant and positive.
Watch it explained
- How it worksHow do births, deaths, immigration, and emigration combine into one growth rate?
- Worked exampleFind the growth rate for a country with 22 births, 8 deaths, 3 immigrants and 5 emigrants per 1000.
- Worked exampleFind next year's population for a city of 500000 people growing at 1.4 percent.
- Common mistakeWhy doesn't a falling growth rate mean a falling population?
Check yourself
End of topic.