By 2025, the biggest cities in the world will not only be bigger, but exceptionally denser. According to a new study, Hong Kong will be the densest megacity on the planet, almost twice as crowded as the runner up. And here in the U.S.—and this might be a real shocker—Los Angeles may be more dense than New York City.

A new study by Bloomberg News looked at 40 urban areas where population growth is expected to reach 4.5 million by 2025, and ranked them by projected population per square mile. One thing to note: Only countries with a 2014 per capita GDP of least $10,000 are on this list—so no cities in China (which the International Monetary Fund lists as a 2013 per capita GDP of $9,884, barely missing the cutoff) or India, famously home some of the most crowded on the planet, are included. In other words, these are only the largest, richest cities (in 2014 terms), ranked.

RankGlobal city, countryPopulation per square mile 2025Population per square mile 1995Estimated 2025 population (000)Population growth in one generation, %
1Hong Kong SAR, China

76,985

57,965

8,160,447

32.8%

2Salvador, Brazil

38,643

19,588

5,216,832

97.3

3Mexico City, Mexico

30,726

21,013

24,580,942

46.2

4Sao Paulo, Brazil

21,068

14,498

23,174,743

45.3

5Singapore, Singapore

20,867

12,524

5,801,022

66.6

6Guadalajara, Mexico

19,579

11,829

5,677,822

65.5

7Brasilia, Brazil

18,974

8,682

4,933,286

118.6

8Santiago, Chile

18,655

13,064

7,088,879

42.8

9Rio de Janeiro, Brazil

17,463

13,044

13,621,289

33.9%

10Jiddah, Saudi Arabia

16,067

6,769

5,221,695

137.4

Still, this data is very interesting, especially when you look at the cities where population is expected to skyrocket. While Hong Kong is ranked to be the densest city by far, it doesn't have to grow all that much to get there—it's already packing plenty of people per square mile. Take a look at the next biggest city, Salvador, Brazil, which will see growth of almost 100 percent, doubling its population within one generation. That's some serious transformation ahead.

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Two cities in Saudi Arabia—Jiddah and Riyadh—could see growth of more than 135 percent. No wonder those cities are all embarking upon massive infrastructure projects like brand-new subways—they're gonna need a way to get all those people around.

While the trend story accompanying the data is all about how cities are expanding underground with projects like New York City's Lowline, like some kind of ubercivilization of mole men, the truth is that most cities won't have to go subterranean in order to add density. If you look at the American cities on the list, for example—which don't even come into consideration until #27—the reason that places like Los Angeles, San Francisco and Miami are going to usurp New York City is because they have 1) lots of room to grow and 2) an economic climate that's bullish on adding buildings. This is actually the case for most U.S. cities on the list.

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LA's current sprawl means there are plenty of short buildings and vacant lots ripe for higher-rise development. (Although, fun fact: the Los Angeles County-Orange County corridor is already the most densely populated urban area in the U.S.) If you think that's interesting, here's something that will also bend your mind: According to the study, Montreal and Toronto, not normally thought of as towering metropolises, could both wind up denser than any city in the U.S. [Bloomberg News]