You can fit a hell of a lot of living space in 1 acre if you use it well. Decent mass transit also increases the effective space of the city from a job perspect

You can fit a hell of a lot of living space in 1 acre if you use it well. Decent mass transit also increases the effective space of the city from a job perspective London density could easily be that of Westminster (the borough with large open areas such as Hyde Park, Green Park, St James Gardens, most of Regents Park etc -- imagine all London boroughs having that much public open space). Westminster isn't even particularly dense, but that alone would allow an extra 6 million people to live in Greater London. Go for Manhattan density and that's 3 times Westminster, allowing 45 million in Greater London.