While the United States population density is about 90 people per square mile, most people live in cities, which have a much higher density. Even among cities, density values can vary considerably from one city to another.
Population density allows for broad comparison of settlement intensity across geographic areas. In the U.S., population density is typically expressed as the number of people per square mile of land area. The U.S. value is calculated by dividing the total U.S. population (316 million in 2013) by the total U.S. land area (3.5 million square miles).
In a broad sense, this number tells us how many people would live within one square mile if the U.S. population were evenly distributed across its land area. In reality, however, we know that population is not evenly distributed across space. People tend to cluster in cities, and those who live in rural areas are spread out across a much more sparsely populated landscape.
While the population density inside U.S. cities (about 1,600 people per square mile) is much greater than that of unincorporated areas (about 35 people per square mile), density levels still vary quite a bit from city to city, and even across different neighborhoods within a single city.
For example, New York City’s density value (almost 28,000 people per square mile) is notably higher than that of Los Angeles (about 8,300 people per square mile). Within New York City, we find that residential neighborhoods with high-rise apartments and condos are much more densely populated than neighborhoods that consist of detached, single-family housing units. The Co-op City neighborhood in the Bronx has density levels that are greater than 33,000 people per square mile. Its high-rise development pattern differs considerably from that of Staten Island, where the overall density is about 8,000 people per square mile
When comparing population density values for different geographic areas, then, it is helpful to keep in mind that the values are most useful for small areas, such as neighborhoods. For larger areas (especially at the state or country scale), overall population density values are less likely to provide a meaningful measure of the density levels at which people actually live, but can be useful for comparing settlement intensity across geographies of similar scale.
For more information on this topic, see the report, Population Trends in Incorporated Places: 2000 to 2013, as well as Patterns of Metropolitan and Micropolitan Population Change: 2000 to 2010.
Source: Census Bureau, written by Darryl Cohen