WASHINGTON (Reuters) - Figuring out the number of American residents got as easy as pi on Tuesday as the United States touched a rare mathematical and demographic milestone.

The Census Bureau said that the United States reached 314,159,265 residents, or the mathematical ratio pi times 100 million, shortly after 2:29 p.m. EDT (1829 GMT).

Pi, or 3.14159265, is the mathematical constant that is the ratio of a circle's circumference to its diameter. It often is approximated as 22/7.

"This is a once in many generations event ... so go out and celebrate this American pi," Census Bureau Chief Demographer Howard Hogan said in a statement.

The pi milestone is "very meaningful to mathematical statisticians," the statement said.

As of 3:54 p.m. EDT (1954 GMT), the bureau's population clock showed a U.S. population of 314,159,667. There were 7,032,793,077 people in the world. http://www.census.gov/main/www/popclock.html

(Reporting by Ian Simpson; Editing by Sandra Maler)