Urban areas in the United States are defined by the U.S. Census Bureau as contiguous census block groups with a population density of at least 1,000 /sq mi (390 /km2) with any census block groups around this core having a density of at least 500 /sq mi (190 /km2). Urban areas are delineated without regard to political boundaries. The census has two distinct categories of urban areas. Urbanized Areas have populations of greater than 50,000, while Urban Clusters have populations of less than 50,000. An urbanized area serves as the core of a metropolitan statistical area, while an urban cluster serves as the core of a micropolitan statistical area.