In the 2001 census Southampton and Portsmouth were recorded as being parts of separate urban areas, however by the time of the 2011 census they had merged to become the sixth largest built-up area in England with a population of 855,569. This built-up area is part of the metropolitan area known as South Hampshire, which is also known as Solent City, particularly in the media when discussing local governance organisational changes. With a population of over 1.5 million this makes the region one of the United Kingdom's most populous metropolitan areas.

What other city merged with Southampton after the 2001 census?