Census Tracts are geographical regions defined and maintained by the U.S. Census Bureau for the purpose of collecting data. Tracts are subdivisions of counties. Tract boundaries are updated prior to each decennial Census, and are based on the number of people living in the tract–the denser the population, the smaller the geographical footprint of the tract, and vice versa. Census Tracts have a population of anywhere from 1,200 to 8,000 people, with 4,000 being the optimum number. There are about 40,000 ZIP codes in the United States, and about 73,000 Census Tracts.

To read more about Census Tracts, see the Census Bureau’s glossary entry.