How Does the Census Bureau Measure the Future Population?

Have you ever wondered how the Census Bureau projects the future population of the United States? We start with a few basic questions: How many people currently reside in the United States? How many births will there be? How many people will die? How many people will move to the United States? How many people will move out of the United States? The answers to these questions are the basis of our population projections. Once we answer these questions, calculating the future population is simple. We just add the number of births and people who move in to the current population, subtract the number of deaths and those who leave, and see how the demographic future unfolds!

The Census Bureau projects the size and characteristics (age, sex, race, and Hispanic origin) of the future population using the most current estimates of the U.S. population. Projections are produced for future years one year at a time based on the components of population change – births, deaths, and international migrants. Assumptions about the future number of births, deaths, and international migrants are based on past trends in these components.

To project deaths, we look at death rates, that is, the number of deaths per thousand population by age. Since these rates have been falling in recent years, we examine recent trends and assume that similar trends will continue into the future. We must decide, for example, to what extent recent reductions in old age mortality will continue. Similarly, for births, we look at fertility rates, that is, the number of births per women in the childbearing ages. Rates have been falling in recent years. Will these downward trends continue? We try to find a balance between these recent trends and what are likely to be the longer-term trends.

International migration is projected as the difference between the number of immigrants who come to the United States and the number of emigrants who leave from the United States each year. Levels of international migration over the past several decades are evaluated to determine how many migrants can be expected to come in the future. The level of international migration increased steadily during the 1980s and 1990s, reaching an annual peak of 1.2 million in 2001. Over the course of the first decade of the 2000s, the number of net international migrants decreased, reaching a level around 725,000 in 2011. We must decide whether more recent short-term declines in international migration will persist into the future, or if the levels of net international migration will ultimately continue to increase following the longer-term trends. Again, we try to find a balance between these recent trends and the longer-term trends.

Once assumptions about fertility, mortality, and international migration have been made, we can produce the population projections. For each projected year, we begin with the population for the previous year and age that population forward one year, so that newborns are now 1 year old, 1 year olds are now 2, etc. Next, we calculate the number of births by applying the projected fertility rates to the female population in that year. We then add the projected number of births and immigrants to the population at the beginning of that year. Deaths for that year are calculated by applying the projected mortality rates to the population in that year. The projected number of deaths and emigrants are then subtracted from the population. The result is the projected population for the next year. This process is repeated for each year of the projection series, by sex, race, and Hispanic origin, typically for a total of 50 years. Through repeating this process, we see how the future will unfold from a demographer’s perspective.

The Census Bureau’s population projections are used by researchers, policymakers, businesses, and other government agencies for a variety of purposes. A topic of great interest is the aging of the population. Projections of the old-age population are of particular interest for those assessing government programs such as Medicare and Social Security. Projections of the working-age population, typically between the ages of 20 and 64, are of interest to businesses and service providers attempting to evaluate future demand for their products and services as well as the means of supplying those goods. Projections of births and the population under the age of 18 are of interest to educators tasked with planning for future demands on the education system.

For more information about the Census Bureau’s population projections, including the methodology used to produce our most recent projections and results, please visit:  //www.census.gov/population/projections/.