The Demographia estimate represents one of the first efforts to quantify the extent and population of the "built up" portions of the London area. For years, the London area has caused demographers difficulty in estimating the urbanized area. This complexity is the result of London's "green belt," which surrounds what is now the Greater London area. Some researchers, such as Kenworthy and Laube (who developed urbanized area estimates for a number of areas around the world) simply considered Greater London to be the urbanized area. There are difficulties with this approach, not least the fact that Greater London had reached its current population by 1911. At its peak (1939), Greater London's population reached 8.6 million, approximately 1.5 million above its present level (1998). In fact, the London area has grew significantly during the 20th century, with the two rings of counties outside the Green Belt rising from 4.2 million to 10.4 million. London's Green Belt caused development to "leap frog" to areas beyond.

The Demographia estimate uses the 1990 US Census Bureau urbanized area methodology, which classifies any municipal jurisdiction as a part of an urbanized area if it is contiguous and has a population density of 1,000 per square mile.

The effect of this methodology is to consider the Green Belt as a part of the urbanized area, since all parts of it are within jurisdictions (local authority areas or London boroughs) with a population density of more than 1,000 per square mile. This is consistent with the treatment of large parks in other urbanized areas, such as the Ville de Paris' peripheral parks, Griffith Park in Los Angeles and other urban reserves

Inner London has a population density of 22,224 per square mile, down 42 percent from its peak of 38,476 reached in 1901. Outer London's population density is 9,121, near its 1939 peak. The outer suburbs contain 5.1 million people spread over 2,322 square miles for a density of 2,186 (Table).

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