About 1.5 million people live in Manhattan, an imposing number that's larger than the entire populations of Phoenix, Dallas and San Francisco. More impressive, though, is what happens on the island by day: So many commuters come in (and so few residents commute out for work) that Manhattan's population nearly doubles in size.
This latter number – 3,083,102, to be precise, according to American Community Survey data collected between 2006 and 2010 – is in some ways an even more important one than the population figure we typically affix to places. If Manhattan ever needs to evacuate by day during a disaster, the city has to figure out what to do with all 3 million of those people. The city's transportation planners are responsible for every one of them, whether they live in New York or not. And anyone who does business in a service industry on the island – from lunch counters to dry cleaners to department stores – cares a lot more about how many people pass through during the day than who passes out in Manhattan at night.