How the U.S. Population Clock Works

On October 17, 2006, the official U.S. Population Clock at the U.S. Department of Commerce building in Washington, DC, passed 300 million people, just 39 years after passing the 200 million mark on November 20, 1967. A fascinating and thought provoking thing to watch, the U.S. Census Bureau's online U.S. Population Clock appears to be reporting a running, real time headcount of the exact number of people residing in the United States at any given moment. But, does the Census Bureau actually know the instant someone is born, dies, leaves or migrates into to the United States? How does the U.S. Population Clock really work?

A Carefully Calculated Estimate

Of course, the Census Bureau's U.S. Population Clock is not all-knowing. Hospitals do not call the Census Bureau every time a baby is born or someone dies. In reality, the population clock is showing a highly accurate, up-to-the-minute estimate of the U.S. population based on historic monthly estimates of population growth rates. This estimate combines historic monthly rates of births, deaths, migration from other countries and movement of members of the U.S. Armed Forces into and out of the United States. Assuming that this estimate will produce a daily change in population that remains constant throughout the current month, the Population Clock applies the estimate to produce an apparent running count of the U.S. population.

From its experience dating back to 1790, the Census Bureau has learned that monthly population growth rates vary during the year. For example, death rates during the winter months are traditionally higher than the death rates in warmer months. Applying these monthly variations in net population change to its calculations helps the Population Clock display a more accurate estimated total population.

For example, during November 2011, the U.S. Population Clock used the following estimates in producing its estimated running total:

• One birth every 8 seconds
• One death every 12 seconds
• One net international immigrant (including military personnel) every 43 seconds
• Resulting in a net gain of one person every 16 seconds

How the Census Bureau Estimates the Population

In producing its estimates of the U.S. population, the Census Bureau uses a demographic accounting method proven over time to produce accurate results. Using data provided by the National Center for Health Statistics, the Census Bureau adds the number of children who were born and subtracts the number of people who have died since the last decennial census.

The estimate also takes into account in- and out-migration, the number of people moving in and out of the United States such as international migrants, people in the military and U.S. residents who move to other countries. Data reported by the U.S. Customs and Immigration Service and the American Community Survey is used to calculate net international migration.

The various branches of the military and the Department of Defense provide information on the movement of military personnel leaving and re-entering the United States. In addition, the U.S. Bureau of Prisons provides data on the current inmate populations of federal, state and local prisons.

Also See: How Prison Cities Cash in on the Census

Who is Included in U.S. Population Estimates?

When the Census Bureau makes its national population estimates, they include residents of the 50 states and the District of Columbia. The estimates do not include residents of the Commonwealth of Puerto Rico and residents of the Island areas under U.S. sovereignty or jurisdiction, mainly American Samoa, Guam, United States Virgin Islands, and the Commonwealth of the Northern Mariana Islands. Also excluded from the U.S. population estimates are members of the U.S. Armed Forces stationed overseas, as well as civilian U.S. citizens whose usual place of residence is outside the United States. The Census Bureau defines a "resident" of specific area as a person who is "usually resident" in that area.

Are Illegal Immigrants Included?

In an often-questioned and criticized practice, the U.S. Census Bureau does - as required by law -- include illegal or undocumented immigrants in both its population estimates and actual counts of population resulting from each decennial census.

Also See: Should the US Census Count Illegal Immigrants?