The World's Biggest Cities

Urban Agglomerations Worldwide is an interactive map which shows the population size of the world's largest cities. The map uses data from the United Nations to show the size of all the urban agglomerations around the world with a population over 300,000. 

On the map the size and color of the markers show the population size of the city. You can also hover over the circles on the map to discover the actual city population. According to the United Nations data (from 2018) the largest 'urban agglomeration' in the world is Tokyo, with a population of over 37 million.The next largest city is Delhi, with a population over 30 million, closely followed by Shanghai with a population of over 27 million.

For the purposes of the United Nations data set an 'urban agglomeration' is defined as a 'contiguous urban area or built-up area that delimits the city’s boundaries'. Obviously the size of a city's population is partly defined by where you draw its boundaries. For example the top 5 megacities with the largest populations according to Wikipedia are:

1. Tokyo (38,140,000)
2. Shanghai (34,000,000)
3. Jakarta (31,500,000)
4. Delhi (27,200,000)
5. Seoul (25,600,000)

Jakarta (3rd on Wikipedia) is ranked on this map as only the 31st largest city by population. Seoul (5th on Wikipedia) is 35th.

Maps (or lists) which rank cites by population therefore often turn into discussions about how you define the boundaries of the ranked cities.Which is why I think it usually makes more sense to map the density of the world's population by area.

You can view a very granular visualization of global population density on The Pudding's Human Terrain map. This interactive visualization shows population density across the globe using 3D population pyramids. 

The taller a pyramid block on The Pudding map then the more people live there.The Pudding has also used its own map to explore in more detail the pattern of population density around the world. In Population Mountains The Pudding examines how unevenly the world is populated and how and why population density can take different forms in different parts of the world.

Data: 

• The Urban Agglomerations Map uses United Nations data - (File 12  under the Urban Agglomerations tab)
• The European Commission's Global Human Settlement Layer (GHSL) also provides spatial information on the physical size of human settlements and on the population sizes of those settlements.