The population of New York City proper, for example, is 8.6 million, according to the latest U.S. Census Bureau data from 2017. But the population of the city’s metropolitan area, also known by the U.S. Census Bureau as the metropolitan statistical area, is 19.5 million. This metro area stretches from the tip of Long Island to Allentown, Pennsylvania (191 miles to the west), and from Tuckerton, New Jersey, to Kingston, New York, a 160-mile drive to the north.
In many cases, there is a third category, called the urban agglomeration, which is defined by the U.N. as the contiguous built-up area with urban density that stretches beyond the city boundaries across administrative zones over county and state lines. A suburb located in a neighboring state that is close enough for residents to commute to and from a city would not be considered part of the urban agglomeration but would often be considered part of a city’s metropolitan area.
Based on the U.N. data, only two cities in the Western Hemisphere make the top 10 world’s most populous cities: Mexico City, with its metropolitan area population size of 21.6 million and São Paulo, with a population size of 21.7 million. Seven of the top 10 megacities are in Asia, while Cairo’s metro area is on the list at No. 7 with 20.1 million.
In order to maintain an apples-to-apples comparison of city population sizes, we are relying solely on U.N. estimates.