To expand:
All of NYC - 28,053/mi^2 Manhattan alone - 72,033/mi^2 London - 13,410/mi^2
Bear in mind that's using the legal definition of Paris which excludes many of its suburbs.
All of NYC - 28,053/mi^2 Manhattan alone - 72,033/mi^2 London - 13,410/mi^2
Paris: density 55,673/mi^2
SF: density 17,246/mi^2
(source: wikipedia)