A megacity is generally defined as an urban settlement or urban agglomeration with at least 10 million inhabitants. The term describes population size rather than administrative status, geographical area or economic importance. Under the United Nations World Urbanization Prospects 2025 framework, Jakarta is the world's most populous city, with nearly 42 million inhabitants, making it the largest megacity by population under that methodology. Dhaka follows with almost 40 million, while Tokyo has about 33 million.
These figures demonstrate why the largest megacity should not automatically be assumed to be Tokyo, even though Tokyo was historically the most commonly cited answer. The identity of the largest megacity depends partly on the statistical definition of an urban settlement and the year of the population estimate.
The word megacity combines mega, meaning very large, with city. It became widely used in urban geography to describe exceptionally large metropolitan concentrations, particularly as rapid urbanisation produced settlements containing more than ten million people. The ten million threshold is a classification convention rather than a physical or social boundary. Crossing from 9.9 million to 10 million does not suddenly transform the settlement's infrastructure or social structure.
The category is useful because it allows researchers to identify the world's very largest urban populations and study their common challenges. In 2025 the United Nations reported 33 megacities, compared with only 8 in 1975, illustrating the enormous growth of large urban settlements during the modern period.
Jakarta's position in the 2025 United Nations statistics reflects the scale of its wider urban settlement rather than simply the population inside the central municipal jurisdiction. The Jakarta metropolitan region has grown through extensive development across neighbouring administrative territories. Housing, employment, transport and infrastructure have increasingly connected these settlements into a single functional urban system. This process is typical of megacities. Their growth often produces urban corridors extending far beyond the historic centre, with suburban and peri urban areas becoming closely integrated with the central city. Consequently, a megacity should be understood as an urban system rather than merely a downtown area or a single municipal government.
Tokyo provides an important historical comparison. Tokyo was widely regarded as the world's largest urban agglomeration for decades, and older United Nations urban agglomeration estimates placed its population at approximately 37 million in 2025. The revised World Urbanization Prospects 2025 approach produced a different result, placing Tokyo at approximately 33 million and Jakarta at nearly 42 million. This illustrates the importance of statistical methodology. Population estimates from different sources should not be combined casually because their geographical boundaries may differ. A ranking is meaningful only when the places are measured using sufficiently comparable definitions, dates and methods.
Megacities are characterised by enormous concentrations of housing, employment, infrastructure, transportation and economic activity. They often contain multiple business districts rather than a single central business district, extensive commuter networks, major airports or ports, universities, industrial areas, informal settlements and high value commercial zones. Their internal structure can be extremely unequal. Wealthy neighbourhoods may have advanced infrastructure and high property values while other districts experience inadequate housing, congestion or limited access to services. Therefore, the label megacity describes population scale but says little by itself about living standards, wealth distribution, governance quality or urban form.
The growth of megacities results from several interacting processes. Natural increase can contribute when birth rates exceed death rates, although fertility patterns vary greatly between countries. Migration can add large numbers of residents as people move towards employment, education, services and perceived opportunities. Economic concentration encourages firms and workers to locate close to one another, generating agglomeration economies. Transport improvements allow urbanisation to spread outward while maintaining links to central employment areas. Administrative expansion can also incorporate previously separate settlements into a wider statistical urban area. The relative importance of these mechanisms differs between individual megacities.
Megacities create major infrastructure requirements because enormous populations depend on interconnected systems. Water treatment, sewage, electricity, public transport, roads, drainage, solid waste management, telecommunications, schools, hospitals and emergency services must operate at metropolitan scale. High population concentrations can make infrastructure provision efficient in some respects because many people are served within a relatively small area. They can also make failures consequential because disruption to a major transport corridor, water system or power network can affect millions of people. Flooding, heat, air pollution, earthquakes, tropical cyclones and sea level rise can create additional risks depending on location.
The significance of the world's largest megacity extends beyond its numerical ranking. Megacities are major centres of national and international economic activity and are laboratories for urban planning, transportation, environmental management and public service delivery. Their growth demonstrates that urbanisation is not simply the construction of larger cities but the formation of increasingly complex metropolitan systems. The United Nations expects urban populations to continue increasing, with a large share of future global population growth occurring in cities and towns. The largest megacity can therefore change over time as demographic growth, urban expansion and statistical methodology alter the measured size of competing urban regions.
