US Census Bureau
The United States Constitution (Article 1, Section 2) mandates a decennial census for the purpose of Congressional apportionment. The first Census was conducted in 1790 and counted only heads of household. Over time the Census has expanded to count every person in the U.S. including information on their age, race, and ethnicity as well as a variety of social and economic characteristics. While there are undoubtedly portions of the population that go uncounted, it is the closest we have to complete demographic and economic data on the U.S. population. More information can be found on the Census Bureau's website.