A megacity is a very large city, typically with a population of more than 10 million people. The United Nations Department of Economic and Social Affairs (UN DESA) in its 2018 "World Urbanization Prospects" report defines megacities as urban agglomerations with over 10 million inhabitants. A University of Bonn report holds that they are "usually defined as metropolitan areas with a total population of 10 million or more people". Elsewhere in other sources, from five to eight million is considered the minimum threshold, along with a population density of at least 2,000 per square kilometre. The terms conurbation, metropolis, and metroplex are also applied to the latter.

The total number of megacities in the world varies between different sources and their publication dates. The world had 33 according to UN DESA (in 2018), 25 according to the OCDE, 45 according to CityPopulation.de (in 2023), and 44 according to Demographia (in 2023). A good percentage of these urban agglomerations are in China and India. The other four countries with more than one megacity are Brazil, Japan, Pakistan, and the United States. African megacities are present in Nigeria, Egypt, South Africa, Angola and the DRC; European megacities are present in Russia, France, the United Kingdom, and Turkey (also in Asia); megacities can be found in Latin America in the countries of Brazil, Mexico, Colombia, Peru, and Argentina.

Some sources identify the Greater Tokyo Area as the largest megacity in the world, while some others give the title to the Pearl River Delta in China.

Since, presently, urban data are based on arbitrary definitions that vary from country to country and from year or census to the next, making them difficult to compare, an Urban Metric System (UMS) has been conceived that could correct the problem, since it allows computing the urban area limits and central points, and it can be applied in the same way to all past, present and future population and job distributions.

It is based on vector field calculations obtained by assuming that, in a given space, all inhabitants and jobs exert the same attractive force A and repulsive force R. The net force (A - R) exerted by each inhabitant or job is given by [1/(1 + d)] - [1/( β + d/2)], where d = distance and β is the only parameter.

UMS distinguishes the following types of urban areas(including "patropolises" that are tantamount to "megacities"), each type corresponding to a given value of β:

	Urban area	Distance at which the attractive force = the repulsive force	Value of β
1	Central city	10 km	6
2	Agglomeration	20 km	11
3	Metropolis	40 km	21
4	Patropolis	80 km	41
5	Megalopolis	160 km	81
6	Urban system	320 km	161
7	Urban macrosystem	640 km	321
8	Continental system	1,280 km	641
9	Intercontinental system	2,560 km	1,281
10	World system	5,120 km	2,561

UMS has been applied to some Canadian cases since 2018, but the data presented in this article are still based on the various existing national definitions, which are disparate.

Megacity	Image	Country	Region	Estimated population
				Citypopulation.de (2024)	Demographia (2023)	UN DESA (2018)	OECD (2020)
Bangalore		India	South Asia	14,200,000	15,257,000	11,440,000	14,253,019
Bangkok		Thailand	Southeast Asia	21,200,000	18,884,000	10,156,000	18,601,400
Beijing		China	East Asia	21,200,000	18,883,000	19,618,000	20,738,738
Bogotá		Colombia	South America	10,400,000	10,252,000	10,574,000	10,544,590
Buenos Aires		Argentina	South America	16,700,000	15,748,000	14,967,000	14,590,526
Cairo		Egypt	North Africa	22,500,000	22,679,000	20,076,000	27,925,433
Changsha		China	East Asia	11,000,000	5,065,000	4,345,000	4,009,195
Chengdu		China	East Asia	17,300,000	15,016,000	8,813,000	9,768,500
Chennai		India	South Asia	12,600,000	11,570,000	10,456,000	11,528,915
Chongqing		China	East Asia	10,900,000	12,653,000	14,838,000	8,913,804
Delhi		India	South Asia	34,600,000	31,190,000	28,514,000	33,495,554
Dhaka		Bangladesh	South Asia	22,500,000	19,134,000	19,578,000	22,762,988
Dongguan		China	East Asia	Combined with Guangzhou	10,753,000	7,360,000	Combined with Guangzhou
Guangzhou		China	East Asia	70,100,000	27,119,000	12,638,000	16,650,322
Hangzhou		China	East Asia	13,900,000	9,618,000	7,236,000	9,013,951
Ho Chi Minh City		Vietnam	Southeast Asia	13,900,000	14,953,000	8,145,000	14,247,593
Hyderabad		India	South Asia	11,400,000	9,797,000	9,482,000	9,706,886
Istanbul		Turkey	Europe, West Asia	15,900,000	14,441,000	14,751,000	14,693,269
Jakarta		Indonesia	Southeast Asia	29,200,000	35,386,000	10,517,000	32,513,588
Jieyang		China	East Asia	Combined with Shantou	2,516,000	-	13,891,202
Johannesburg		South Africa	Southern Africa	14,600,000	15,551,000	5,486,000	9,497,036
Karachi		Pakistan	South Asia	20,900,000	20,249,000	15,400,000	18,916,709
Kinshasa		DR Congo	Central Africa	15,600,000	13,493,000	13,171,000	10,077,694
Kolkata		India	South Asia	17,700,000	21,747,000	14,681,000	24,106,859
Lagos		Nigeria	West Africa	20,700,000	14,540,000	13,463,000	12,642,198
Lahore		Pakistan	South Asia	14,500,000	13,504,000	11,738,000	15,696,939
Lima		Peru	South America	11,800,000	10,556,000	10,391,000	10,496,389
London		United Kingdom	Europe	14,900,000	10,803,000	9,046,000	13,475,297
Los Angeles		United States	North America	17,200,000	15,587,000	12,458,000	16,206,529
Luanda		Angola	Central Africa	9,350,000	10,914,000	7,774,000	10,212,263
Metro Manila		Philippines	Southeast Asia	27,200,000	24,156,000	13,482,000	27,327,889
Mexico City		Mexico	North America	25,100,000	21,905,000	21,581,000	19,229,491
Moscow		Russia	Europe	19,700,000	17,878,000	12,410,000	17,217,606
Mumbai		India	South Asia	27,100,000	25,189,000	19,980,000	23,435,141
Nagoya		Japan	East Asia	10,500,000	9,439,000	9,507,000	9,853,994
New York City		United States	North America	22,000,000	21,396,000	18,819,000	20,106,617
Osaka		Japan	East Asia	17,700,000	14,916,000	19,281,000	16,866,788
Paris		France	Europe	11,500,000	11,108,000	10,901,000	11,249,025
Rio de Janeiro		Brazil	South America	12,500,000	12,306,000	13,293,000	11,068,999
São Paulo		Brazil	South America	22,100,000	21,486,000	21,650,000	21,671,857
Seoul		South Korea	East Asia	25,100,000	23,225,000	9,963,000	25,199,125
Shanghai		China	East Asia	40,800,000	24,042,000	25,582,000	30,504,083
Shenzhen		China	East Asia	Combined with Guangzhou	17,778,000	11,908,000	Combined with Guangzhou
Surabaya		Indonesia	Southeast Asia	5,900,000	6,556,000	-	10,695,358
Suzhou		China	East Asia	Combined with Shanghai	6,091,000	6,339,000	13,458,006
Taipei		Taiwan	East Asia	9,950,000	9,662,000	-	10,048,037
Tehran		Iran	West Asia	16,500,000	13,382,000	8,896,000	13,563,316
Tianjin		China	East Asia	11,500,000	10,047,000	13,215,000	8,963,397
Tokyo		Japan	East Asia	41,000,000	37,785,000	37,468,000	36,697,549
Wuhan		China	East Asia	12,200,000	10,353,000	8,176,000	8,947,812
Xiamen		China	East Asia	14,900,000	5,253,000	3,585,000	4,261,898
Xi'an		China	East Asia	12,800,000	12,211,000	7,444,000	6,818,858
Zhengzhou		China	East Asia	9,800,000	11,068,000	4,940,000	6,381,637

The term "megacity" entered common use in the late 19th or early 20th centuries; one of the earliest documented uses of the term was by the University of Texas in 1904. Initially the United Nations used the term to describe cities of 8 million or more inhabitants, but now uses the threshold of 10 million. In the mid 1970s the term was coined by urbanist Janice Perlman referring to the phenomenon of very large urban agglomerations.

In 1800, only 3% of the world's population lived in cities, a figure that rose to 47% by the end of the twentieth century. In 1950, there were 83 cities with populations exceeding one million; by 2007, this number had risen to 468. The UN forecasts that today's urban population of 3.2 billion will rise to nearly 5 billion by 2030, when three out of five, or 60%, of people will live in cities. This increase will be most dramatic on the least-urbanized continents, Asia and Africa. Surveys and projections indicate that all urban growth over the next 25 years will be in developing countries. One billion people, almost one-seventh of the world's population, now live in shanty towns. In many poor countries, overcrowded slums exhibit high rates of disease due to unsanitary conditions, malnutrition, and lack of basic health care. By 2030, over 2 billion people in the world will be living in slums. Over 90% of the urban population of Ethiopia, Malawi and Uganda, three of the world's most rural countries, already live in slums.

By 2025, Asia alone will have at least 30 megacities, including Mumbai, India (2015 population of 20.75 million people), Shanghai, China (2015 population of 35.5 million people), Delhi, India (2015 population of 21.8 million people), Tokyo, Japan (2015 population of 38.8 million people) and Seoul, South Korea (2015 population of 25.6 million people). In Africa, Lagos, Nigeria has grown from 300,000 in 1950 to an estimated 21 million today.

For almost five hundred years, during the period of the Republic and later of the Empire, Rome was the largest, wealthiest, and most politically important city of the ancient world, rulling over Europe, Western Asia and Northern Africa. It is often stated that its population passed one million people by the end of the 1st century BC, however, its sometimes debated about whether the population actually reached such a large size. Rome's population started declining in 402 AD when Flavius Honorius, Western Roman Emperor from 395 to 423, moved the government to Ravenna and Rome's population declined to a mere 20,000 during the Early Middle Ages, reducing the sprawling city to groups of inhabited buildings interspersed among large areas of ruins and vegetation.

Baghdad was likely the largest city in the world from shortly after its foundation in 762 AD until the 930s, with some estimates putting its population at over one million. Chinese capital cities Chang'an and Kaifeng also experienced huge population booms during prosperous empires. According to the census in the year 742 recorded in the New Book of Tang, 362,921 families with 1,960,188 persons were counted in Jingzhao Fu (京兆府), the metropolitan area including small cities in the vicinity of Chang'an. The medieval settlement surrounding Angkor, the one-time capital of the Khmer Empire which flourished between the 9th and 15th centuries, could have supported a population of up to one million people.

From around 1825 to 1918 London was the largest city in the world, with the population growing rapidly; it was the first city to reach a population of over 5 million in 1900. In 1950, New York City was the only urban area with a population of over 10 million. Geographers had identified 25 such areas as of October 2005, as compared with 19 megacities in 2004 and only nine in 1985. This increase has happened as the world's population moves towards the high (75–85%) urbanization levels of North America and Western Europe.

Since the 2000s, the largest megacity has been the Greater Tokyo Area. The population of this urban agglomeration includes areas such as Yokohama and Kawasaki, and is estimated to be between 37 and 38 million. This variation in estimates can be accounted for by different definitions of what the area encompasses. While the prefectures of Tokyo, Chiba, Kanagawa, and Saitama are commonly included in statistical information, the Japan Statistics Bureau only includes the area within 50 kilometers of the Tokyo Metropolitan Government Offices in Shinjuku, thus arriving at a smaller population estimate. A characteristic issue of megacities is the difficulty in defining their outer limits and accurately estimating the populations.

Another list defines megacities as urban agglomerations instead of metropolitan areas. As of 2021, there are 28 megacities by this definition, like Tokyo. Other sources list Nagoya and the Rhein-Ruhr as megacities.

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According to the United Nations, the proportion of urban dwellers living in slums or informal settlements decreased from 47 percent to 37 percent in the developing world between 1990 and 2005. However, due to rising population, the absolute number of slum dwellers is rising. The majority of these are located in informal settlements which often lack sufficient quality housing, sanitation, drainage, water access, and officially recognized addresses. The increase in informal settlement population has been caused by massive migration, both internal and transnational, into cities, which has caused growth rates of urban populations and spatial concentrations not seen before in history. These issues raise problems in the political, social, and economic arenas. People who live in slums or informal settlements often have minimal or no access to education, healthcare, or the urban economy.

As with any large concentration of people, there is usually crime. High population densities often result in higher crime rates, as visibly seen in growing megacities such as Karachi, Delhi, Cairo, Rio de Janeiro, and Lagos.

Megacities often have significant numbers of homeless people. The actual legal definition of homelessness varies from country to country, or among different entities or institutions in the same country or region.

In 2002, research showed that children and families were the largest growing segment of the homeless population in the United States, and this has presented new challenges, especially in services, to agencies. In the US, the government asked many major cities to come up with a ten-year plan to end homelessness. One of the results of this was a "Housing first" solution, rather than to have a homeless person remain in an emergency homeless shelter it was thought to be better to quickly get the person permanent housing of some sort and the necessary support services to sustain a new home. But there are many complications with this kind of program and these must be dealt with to make such an initiative work successfully in the middle to long term.

Traffic congestion is a condition on road networks that occurs as use increases, and is characterized by slower speeds, longer trip times, increased pollution, and increased vehicular queueing. The Texas Transportation Institute estimated that, in 2000, the 75 largest metropolitan areas experienced 3.6 billion vehicle-hours of delay, resulting in 5.7 billion U.S. gallons (21.6 billion liters) in wasted fuel and $67.5 billion in lost productivity, or about 0.7% of the nation's GDP. It also estimated that the annual cost of congestion for each driver was approximately $1,000 in very large cities and $200 in small cities. Traffic congestion is increasing in major cities and delays are becoming more frequent in smaller cities and rural areas.

Urban sprawl, also known as suburban sprawl, is a multifaceted concept, which includes the spreading outwards of a city and its suburbs to its outskirts to low-density, auto-dependent development on rural land, with associated design features that encourage car dependency. As a result, some critics argue that sprawl has certain disadvantages including longer transport distances to work, high car dependence, inadequate facilities (e.g. health, cultural. etc.) and higher per-person infrastructure costs. Discussions and debates about sprawl are often obfuscated by the ambiguity associated with the phrase. For example, some commentators measure sprawl only with the average number of residential units per acre in a given area. But others associate it with decentralization (spread of population without a well-defined center), discontinuity (leapfrog development), segregation of uses, etc.

Gentrification and urban gentrification are terms for the socio-cultural changes in an area as a result of wealthier people buying property in a less prosperous community. As living costs rise, lower-income residents are forced to move out of the community leading to an increase in average income, which in turn makes the area more desirable to other wealthier property or business owners, further pushing the living costs up. This process also tends to lead to a decrease in average family size in the area. This type of population change reduces industrial land use when it is redeveloped for commerce and housing.

Air pollution is the introduction into the atmosphere of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or damages the natural environment. Many urban areas have significant problems with smog, a type of air pollution derived from vehicle emissions from internal combustion engines and industrial fumes that react in the atmosphere with sunlight to form secondary pollutants that also combine with the primary emissions to form photochemical smog.

The sheer size and complexity of megacities gives rise to enormous social and environmental challenges. Whether megacities can develop sustainably depends to a large extent on how they obtain, share, and manage their energy and material resources. There are correlations between electricity consumption, heating and industrial fuel use, ground transportation energy use, water consumption, waste generation, and steel production in terms of level of consumption and how efficiently they use resources.

Megacities are a common backdrop in dystopian science fiction, with examples such as the Sprawl in William Gibson's Neuromancer, and Mega-City One, a megalopolis of between 50 and 800 million people (fluctuations due to war and disaster) across the east coast of the United States, in the Judge Dredd comic. In Demolition Man a megacity called "San Angeles" was formed from the joining of Los Angeles, Santa Barbara, San Diego and the surrounding metropolitan regions following a massive earthquake in 2010. Fictional planet-wide megacities (ecumenopoleis) include Trantor in Isaac Asimov's Foundation series of books and Coruscant (population two trillion) in the Star Wars universe.

• Economies of agglomeration
• Global city
• List of largest cities
• List of largest cities throughout history
• Megalopolis
• Urban sprawl