Why density numbers vary so widely
Urban density, measured as residents per square kilometer, differs by an order of magnitude between cities built around walking and transit and those built around the car. Dense cores such as central Paris and Manhattan pack tens of thousands of people into each square kilometer, while sprawling auto-oriented regions like greater Los Angeles spread far fewer residents over a similar area.
The figures also depend heavily on where the boundary is drawn. A measurement of a tight historic core gives a very high number, while including suburban counties pulls the average down sharply, which is why density comparisons must specify the area being counted.
What density shapes in a city
Higher residential density supports frequent public transit, walkable shops, and lower per-capita energy use, but it also raises pressure on housing prices and infrastructure. Lower density tends to mean car dependence, longer commutes, and more land consumed per resident.
Comparing cities side by side highlights the tradeoffs planners weigh when deciding whether to build up or build out.