How many people live in the world’s largest city, according to the UN’s 2025 estimate?

How to get close to this number

The largest cities on Earth sit in the thirty-to-forty-million range, so if you know any of the giants — Tokyo, Delhi, Shanghai, Dhaka — you can place the maximum, because the top handful are within a modest factor of each other. The leader is the one most people forget: for decades the textbook answer was Tokyo, and that habit pulls guesses toward the wrong city even when the order of magnitude is right.

The bigger lesson is in the definition. The UN's 2025 revision stopped relying on each country's own city boundaries and started counting the continuous, densely built-up area around each city centre. Jakarta's official statistics had left out dense communities that run straight into it, which is how the same city went from 12 million under the old rules to 41.9 million under the new ones without anyone moving.

The projection in the explanation is the other half of the story: the fast-growing challenger is expected to take the top spot by mid-century, while Tokyo, in an ageing, low-birth-rate country, is projected to shrink. Every major city ranking over the next few decades will be reshuffled by that demographic divergence, and the top three are where it becomes visible first.