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I think you mixed up imperial and metric systems --- San Francisco has 18,451 people/sq mi. Paris has 55,000 people/sq mi. It's almost three times as much.

Not if you include the homeless people! [Ducks]

Paris is almost 3 times denser than San Francisco.

There's got to be a more useful metric than average city density; the 2010 US Census [0] places the 5 densest cities in the US as having a population under 70,000 (yes, I'm quoting Wikipedia). City planners must have a unit more like person-acres or something to get an idea of the actual spread of the density.

[0]: https://en.wikipedia.org/wiki/List_of_United_States_cities_b...

Standard density quotes for cities can be pretty misleading as it often reflects fairly arbitrary divisions. For example while Paris is indeed reasonably dense the oft quoted number is for a fairly small parcel of central Paris.

http://austinzoning.typepad.com/austincontrarian/2008/03/per...