The United States has a total resident population of 309,147,000. It is a very urbanized population, with 81% residing in cities and suburbs as of mid-2005 (the worldwide urban rate was 49%). California and Texas are the most populous states, as the mean center of United States population has consistently shifted westward and southward.

The total fertility rate in the United States estimated for 2009 is 2.05 children per woman, which is roughly the replacement level. However, U.S. population growth is among the highest in industrialized countries, since the vast majority of these have below-replacement fertility rates and the U.S. has higher levels of immigration. The United States Census Bureau shows population increases ranging between 0.85% and 0.89% for the twelve-month periods ending in 2009. Nonetheless, though high by industrialized country standards, this is below the world average annual rate of 1.19%.

People under 20 years of age made up over a quarter of the U.S. population (27.6%), and people age 65 and over made up one-eighth (12.6%) in 2007. The national median age was 36.7 years. Racially, the U.S. has a White American majority. Minorities compose just over one-third of the population (102.5 million in 2007), with Hispanic and Latino Americans and African Americans as the largest minority groups, by ethnicity and race, respectively.

The American population more than tripled during the 20th century—a growth rate of about 1.3% a year—from about 76 million in 1900 to 281 million in 2000. It reached the 200 million mark in 1967, and the 300 million mark on October 17, 2006. Currently, population growth is fastest among minorities as a whole, and according to the Census Bureau's estimation for 2005, 45% of American children under the age of 5 belonged to minority groups.

Hispanic and Latino Americans accounted for almost half (1.4 million) of the national population growth of 2.9 million between July 1, 2005, and July 1, 2006. Immigrants and their U.S.-born descendants are expected to provide most of the U.S. population gains in the decades ahead. Since the liberalization of immigration policy in 1965, the number of first-generation immigrants living in the United States has quadrupled, from 9.6 million in 1970 to about 38 million in 2007. Almost 97% of residents of the 10 largest American cities in 1900 were non-Hispanic whites. In 2006, non-Hispanic whites were the minority in thirty-five of the fifty largest cities.

The Census Bureau projects a U.S. population of 439 million in 2050, which is a 46% increase from 2007 (301.3 million). However, the United Nations projects a U.S. population of 402 million in 2050, an increase of 32% from 2007 (the UN projects a gain of 38% for the world at large). In either case, such growth is unlike most European countries, especially Germany, Russia, Italy, and Greece, or Asian countries such as Japan or South Korea, whose populations are slowly declining, and whose fertility rates are below replacement.

As of 27 April 2010, the United States has 4.53% of the world's population.

The first U.S. census, in 1790, recorded four million Americans. By 2000, this number had grown to 281 million. It is expected to reach 310 million by 2010 and 439 million by 2050.

U.S. population clock hits the 300 million mark

United States population pyramid.