U.S. Population Grows to 320.09 Million. California Still Reigns as Most Populous US State The population of the U.S. is estimated to be 320,090,857 on Jan. 1, 2015, an increase of 0.73 percent when compared to Jan. 1, 2014. The Census Bureau suggests one birth per second in the U.S. in January 2015. by Sumit Passary