The United States Census Bureau keeps time with the population of the U.S. with its population clock, located on its web site. The clock projects the resident population to the current day to a particular hour and is based on previous national estimates of populace. The clock is recalibrated every year after the new set of estimates of released.
Base population, births, deaths, net migrants to the U.S. dictate the basic procedure to create the population estimates. For instance, the December 2009 settings were:
Residents are defined as "usually" residing in the nation: ie: spending most of their time here in a permanent residence; citizens spending spending most of their time at a permanent residence outside of the United States are not included. The resident population includes all 50 states in the union as well as the District of Columbia. The Puerto Rican commonwealth is excluded in the estimates as well as the islands of Guam, American Samoa, the U.S. Virgin Islands, and the Northern Mariana Island commonwealth. The U.S. Armed Forces are excluded when estimating civilian U.S. residents. The United States Census Bureau runs three other estimates for those populations excluded in the population clock.
The population of the United States, in thousands of resident people, historically: