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Tokyo, Japan, with a current urban area (as opposed to "city proper") of 39.6 million people, is currently the largest city in the world.

Is it likely that there ever be a city on Earth that has at least 100 million people in its urbanized area?

If so, approximately when would the first city on Earth to have more than 100 million people in its urbanized area be likely to cross that threshold?

And, which cities in the world are the most likely candidates to cross the 100 million threshold?

BONUS QUESTIONS: What would be the answer to the questions above be if they were applied to the United States of America, rather than to the entire planet?

What is the largest city that could be sustained on Earth assuming no profound breakthroughs in technology from that present today or just around the corner in developmental stages today?

This question assumes nothing counterfactual. It is about the real world Earth and the real world United States, in the foreseeable, predictable future.

Feel free to consider the possibility that a large city arises because cities that currently have separated urbanized areas that are separated by non-urbanized areas grow together to form a single urbanized area.

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    $\begingroup$ To answer this question, it might help to first find out where a single urbanized area consisting of 100 million people would get its food and water from, and what techniques might be used to generate that volume of food within a reasonable distance for delivery and distribution. It's a fair bet that no single urban center will grow that large until we have the logistics of supplying the basic needs of life for that many people in one place ironed out. This information would probably also help to inform us on population density for such a city. $\endgroup$
    – Steve-O
    Commented Feb 17, 2017 at 18:55
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    $\begingroup$ @Steve-O Given that several existing cities are only 2.5 to 3 times smaller than 100 million, it is hard to believe that there is anything fundamental that prohibits cities from getting that large in terms of logistics. We have many relatively compact regions on Earth with several cities that have that many people combined, and the logistics of meeting the basic needs of people in a region aren't that much different than the logistics for a city. $\endgroup$
    – ohwilleke
    Commented Feb 17, 2017 at 19:00
  • $\begingroup$ roughly speak 39 million people is 32,000 sq mi. So find a city with at least 86000 sq mi for these people to live in. There will be your candidate cities. Also you will need a large body of water nearby. theus50.com/fastfacts/area.php Basically the top 10 of this list. If you don't mind starting a new city and melting ice burgs for water a new city in alaska, otherwise a city, or several areas become 1 city, in Texas look like your best options. $\endgroup$
    – cybernard
    Commented Feb 18, 2017 at 3:21
  • $\begingroup$ The city of BosWash has around 50 million, so a hundred million isn't too unreasonable. $\endgroup$
    – Mark
    Commented Feb 18, 2017 at 3:25
  • $\begingroup$ using Boshwash density of 931 people per sq mile you need 108,000 sq miles of land, and you want a city so you need enough spare land to have a few other cities in your US state. The list I provided above gets even shorter at that density. $\endgroup$
    – cybernard
    Commented Feb 18, 2017 at 3:34

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There are already a few Megalopolis's with over 100 million people in them.

In the US there is the Northeast corridor that has 50 million people in it; As it is the largest and the largest cities tend to grow the most that is the one that I would expect to reach 100 million in the US first. If it were to rise in population to have a density of the Atlanta area over the entire area would give it 136 million people. If current growth trends continue (perhaps unlikely) it would reach 100 million around 2080 based on simple population, growth rate, time formula.

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    $\begingroup$ Northeast corridor, however, is not one continuous urban area. Between Washington, Philadelphia, New York and Boston there are still wide expanses with woods and farms. That may change in the future, but, given the present speed of development (I used to live right there :) ), it may take longer than 50 years. $\endgroup$
    – Alexander
    Commented Feb 17, 2017 at 23:44
  • $\begingroup$ +1. It should be noted though that the definition of Megalopolis is fairly large. For example, the European Blue and Golden Bananas are more axes of transportation, communication and poltical development than a "giant city" in the making. Also, do you mind sharing the population growth formula you used? Or at least a source? $\endgroup$
    – Taladris
    Commented Feb 18, 2017 at 4:36
  • $\begingroup$ The formula is just the standard Pert formula from like algebra (here is a link explaining it: coolmath.com/algebra/17-exponentials-logarithms/… ) from 2000 to 2010 the population grew 10.96% which by either dividing by years or just using that growth rate per decade gives 2080 as when it reaches 100 million. So it is assuming a constant growth rate, which is probably not a good assumption. $\endgroup$
    – John_H
    Commented Feb 18, 2017 at 15:45
  • $\begingroup$ The "largest cities tend to grow the most" hypothesis is doubtful in this particular case. The Northeast corridor is one of the slowest growing parts of the U.S. and has an aging population and not much net migration into it. $\endgroup$
    – ohwilleke
    Commented Aug 20, 2019 at 22:04
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The Yangtze river delta is already almost 100 million - consider these large cities are really close to each other:

City Million people
Shanghai........24
Suzhou...........11
Wuxi................6
Changzhou.....5
Nanjing...........15
Nantong...........8
Zhenjiang........3
Yangzhou........5
Jiaxin...............1
Hangzhou........9

Total................87 Million people

It is not outlandish that all these cities could be merged under a single administration that is called "city". While there are a few farms between some of these cities, they won't be there for long. I've met a number of people who commute between these cities on a daily basis.

So, maybe it is already existing? The wikipedia page does say there are 115 million people there, but only 80 or 90 million urban. Or perhaps in a couple years? It really depends on how you count, but if it isn't already, it wont be long.

A side note, not sure if China counts migrant workers in its local census. It if does not, you can increase all these numbers by 1/3.

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    $\begingroup$ See also Pearl River Delta with 63m - 120m people, depending how you count it. $\endgroup$
    – Chris McCormick
    Commented Feb 18, 2017 at 6:46
  • $\begingroup$ There is still a vast amount of open country in both your suggested areas. It's AFRAIR a few hours by bus from Ningbo (across river from Shanghai) to Hangzhou and lots of very pleasant country scenery. Same applies for Chris's Pearl River delta. - So much country scenery than an idiot southbound from Guangzho can overshoot Donguan and end up in an unknown townlet trying to catch a taxi back. What fun :-). $\endgroup$
    – Russell McMahon
    Commented Feb 18, 2017 at 12:44
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    $\begingroup$ Yeah, Hangzhou is the weak part of my argument. But I know people from Shijiazhuang fly to Hangzhou airport when going to Shanghai. Spring Airlines! $\endgroup$
    – Astor Florida
    Commented Feb 18, 2017 at 14:03
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If nothing terrible is going to happen to the world, I'm quite sure there will be a metro area topping 100M residents within the next 50 years. Logistics for such a megacity would be difficult, but not impossible to solve - modern 20M+ cities are doing all right, and what we need to do is scale up a little bit.

Which city will be the first to cross the magic mark is difficult to tell. There are many candidates, like Delhi, Lagos, Jakarta, big question is whether their respective countries will put breaks on population growth anytime soon.

Another way to supersize a city is agglomeration - imagine that Tokyo and Osaka metro areas will merge some time in the future. Resulting megacity will be enormous - we just need to recognize it as one and not two.

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  • $\begingroup$ Tokyo + Osaka is currently about 53 million people plus a bit for the population currently between them. But, Japan has pretty slow population growth right now (it is currently shrinking by 0.2% per year) and is slowly trending towards faster population decline. The total population of Japan is 127.3 million (as of 2013). $\endgroup$
    – ohwilleke
    Commented Feb 17, 2017 at 20:37
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    $\begingroup$ Japan has some deep seated fear and hate of immigrants. If the situation is ever forced, or if they realize they have to start relying on immigrants to make up for their own age gaps in the labor force their population could boom. While not something they at all want, it doesn't mean it can't or won't happen. $\endgroup$
    – user2259716
    Commented Feb 17, 2017 at 22:02
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To answer the main question, seeing how the largest city by area in the world is the New York Metro area with an approximate land area of 8683 sqKM. And the city with the highest population density in the world is Mumbai with 29,650 people per sqKM. It isn't hard to imagine that it could be possible to have a massive and densely packed city of over 100 million people.

A city with those two factors together gets you over 257 million people as is. With an ever growing world population and if you allow for cities to grow and combine together into one bigger city as some do, then 100 million is just a starting point.

Source for area

Source for density

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  • $\begingroup$ The rate at which world population grows is slowing as lots of areas reach demographic transition when they industrialize. The U.S. is one of the few developed countries in the world that does not have stagnant or declining populations. $\endgroup$
    – ohwilleke
    Commented Aug 20, 2019 at 22:07
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The most likely is what Gibbson includes in his early cyberpunk novels, "the sprawl" - large cities near to each other merging (officially or not) into one urban area. There are a few such mergers already in the world, where aside from a road sign you can't tell that you left one and entered another city.

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There isn't a problem with that size. As @ohwilleke stated, it is just a matter of bringing in resources.

The only other consideration is politics. If you took Los Angeles and all of the satellite cities (suburbs) and called them one city, you would have your number.

Heck, Los Angeles and Orange County can almost be considered one system (look at google earth and try to tell me where one city ends and another begins). The city and county lines are just lines on a map and have no functional effect on the residents' daily lives.

However, there are vast differences in politics to the point that such a merger is unlikely.

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