The population clock on the All Things Census page is derived using national-level data from the U.S. Census Bureau, which produces estimates of the country’s total resident population and the components that are the building blocks of demographic change.

Those components include births, deaths and net international migration, computed using data from the Census Bureau and the National Center for Health Statistics. In December 2009, the Census Bureau estimated there was a birth every eight seconds, a death every 12 seconds and one net new immigrant every 37 seconds.

The population clock updates these changes at regular intervals, although in reality, these events are not evenly spaced. There are fewer births on weekends, for example, than on weekdays.

A more detailed report on the methodology of calculating population estimates is on the U.S. Census Bureau website.