Gutierrez said America's growing population is good for the economy. He noted that Japan and some European countries expect to lose population in the next few decades, raising concerns that there won't be enough young people entering the work force to support aging populations.

"This is one more area where we seem to have an advantage," Gutierrez said. "We should all feel good about reaching this milestone."

The U.S. adds about 2.8 million people a year, a growth rate of less than 1 percent.

The Census Bureau counts the population every 10 years. In between, it uses administrative records and surveys to estimate monthly averages for births, deaths and net immigration. The bureau has a "population clock" that estimates a birth every seven seconds, a death every 13 seconds and a new immigrant every 31 seconds. Add it together and you get one new American every 11 seconds.

The U.S. population trails only China and India.

It's not easy estimating the exact number of people in a country the size of the United States. Passel said the Census Bureau has improved its population estimates in the past few years, but it still undercounts illegal immigrants.

There are an estimated 11 million to 12 million illegal immigrants in the U.S. Experts differ on the specifics, but many estimate that more than 1 million of them don't show up in census figures.

"The census clearly misses people," said Passel, a former Census Bureau employee who used to help estimate the undercount. "Having said that, when they crossed 200 million, they were missing about 5 million people. We think the 2000 census missed a lot less than 5 million people."

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