Urban Observatory Compare Cities

"When you imagine people living densely packed, you may think of New York City. But a new report from the Census Bureau shows California is home to the most densely populated metro areas in the United States," according to KPBS.

That ranking takes the population of the entire metropolitan area, and divides it by the land area of the entire metropolitan area, to provide a summary figure for the metro area (not the city).  New York City by itself still leads the nation in terms of population density due to its 8+ million strong residents.

Small area census data reveals that population density varies noticeably from neighborhood to neighborhood, e.g. near Hollywood. Census data reveals that population density varies noticeably from area to area. Small area census data do a better job depicting how where the crowded neighborhoods are. 

In this map, the yellow areas of highest density range from 30,000 to 150,000 persons per square kilometer.  In those areas, if the people were spread out evenly across the area, there would be just 4 to 9 meters between them.  Very high density areas exceed 7,000 persons per square kilometer.  High density areas exceed 5,200 persons per square kilometer. The last categories break at 3,330 persons per square kilometer, and 1,500 persons per square kilometer.