How many people live in a square of land?

Population Guesser shows a square of satellite imagery, 2 to 25 km across, and asks how many people live inside it. The answers run from zero to more than fifteen million, so it can feel like pure guesswork. It isn't: the picture gives away a lot, once you know what to look at.

Most land has nobody on it

On the population map behind the game, about 80% of Earth's land, Antarctica included, has nobody living on it at a 1 km scale. Ice, desert, tundra, mountains and forest take up most of the planet. People crowd into river valleys, coasts and fertile plains instead: almost all of Egypt's population lives on the narrow green strip of the Nile valley and delta, which you can see from space as a ribbon through the desert.

So the first question for any square is not “how many?” but “is anyone here at all?”. Roads, field boundaries and roofs say yes. Unbroken sand, rock or forest usually says almost no one.

The most crowded squares in the game

The densest 2 km square in the game is in eastern Nairobi, with about 302,000 people: roughly 75,000 per square kilometre. Squares in Brazzaville, Istanbul, Hong Kong, New Taipei and Kathmandu follow. At the largest size, a 25 km square over Karachi holds about 15.6 million people, and one over north-west Shanghai about 13.1 million.

City centres can be surprisingly empty

The heart of a city is often not where its people sleep. Here is how many people live per square kilometre in a 1 km square at each city's centre, and in the 10 km square around it.

London's centre point falls in the City, its financial district, which is full of offices and has few residents; the wider square is three times as dense. Tokyo's centre is dominated by the Imperial Palace and its gardens. Dhaka and Mumbai are the opposite: dense right to the middle, and among the most crowded places on Earth.

What to look for in the picture

• Many small roofs packed tight, with narrow lanes: old town centres and informal neighbourhoods. These are the densest places of all.
• Tall buildings: towers cast long shadows and leave open space between them. They can be homes for tens of thousands, or offices that empty at night.
• Big flat roofs and wide yards: factories, warehouses, ports and airports. Lots of building, very few residents.
• A fine, even pattern of houses and gardens: suburbs, typically a few thousand people per square kilometre.
• A patchwork of fields: farmland. Scattered farms in Europe or the Americas mean few people; in the plains of India or Bangladesh, villages are packed in among the fields and a rural square can hold tens of thousands.
• Green doesn't mean empty: in the tropics, trees can hide whole villages.

Where the numbers come from

The game's answers come from the Global Human Settlement Layer, made by the European Commission's Joint Research Centre. It takes national census counts and spreads them over the built-up surfaces that satellites detect, on a grid of cells about 90 m across. Each square's answer is the sum of exactly the cells you see. A square was only kept if a second, independent model, WorldPop, broadly agreed (within a factor of three), because where two good models disagree nobody can be sure of the real number.

Think in multiples

The score depends on how many times too high or too low you are, not on the raw difference, so the useful habit is to settle the order of magnitude first. Is this a square of hundreds, thousands, tens of thousands or millions? Get that right and you are already most of the way to a good score; the details only refine it.