From the U.S. Census:
The U.S. Census Bureau reports that the U.S. population will reach 314,395,013 at noon today, which is the 225th anniversary of the signing of the Constitution on Sept. 17, 1787, in Philadelphia.
Sept. 17 is recognized as Constitution Day and Citizenship Day to commemorate the creation and signing of the supreme law of the land and to honor and celebrate the privileges and responsibilities of U.S. citizenship for both native-born and naturalized citizens. Federal law requires that all schools receiving federal funds hold an educational program for their students on Sept. 17 of each year.
On Sept. 17, 1987, the Census Bureau estimated the U.S. population was 243,636,172 at noon that day, the 200th anniversary of the signing of the Constitution. The first U.S. census in 1790 counted 3,929,214 residents.