The population density of the US is 34 inhabitants/km^2. If you double that, you get to the population density of the state of Georgia, hardly an overpopulated place for world standards. Triple the population and you get to the population density of Spain (quite underpopulated for European standards). If the US had 6 times more inhabitants (1.9 billion), it would be as densely populated as Italy. I know these things don’t scale up linearly and all these regions have different natural resources, etc. But it is interesting to get some perspective to show that nothing essential would change by doubling the population.
Source: Wikipedia, of course
