All eight billion people could stand in an area smaller than Puerto Rico
Pack every person on Earth into a loose crowd, allowing each just under a square metre, and the whole of humanity would fit into about 8,000 square kilometres. That is a little smaller than Puerto Rico's land area of 8,868. The planet only feels crowded because of where people choose to live.
Spread evenly over the planet's entire surface of 510 million square kilometres, land and sea together, 8 billion people works out to about 16 per square kilometre. Count only land, 150 million square kilometres including Antarctica, and it rises to 53. Leave out the frozen continent and it passes 58.
Real life is far lumpier. Some 95% of humanity lives on just 10% of the land. The densest places tend to be city-states, microstates and urban dependencies, small territories whose residents draw on food and resources from outside their borders, which shows why a high density is not the same thing as overpopulation. Deserts, lacking rain for crops, are usually thinly peopled, although cities such as Dubai and Kuwait City have grown quickly. Very dense cities cluster in Asia, alongside places like Lagos, Cairo, Bogotá and Mexico City, but a city's figure depends heavily on whether suburbs and surrounding countryside are counted.
Because the simple ratio of people to land hides so much, geographers use variations. Physiological density divides a population by farmable land, agricultural density does the same with only the rural population, and population-weighted density measures the crowding the average person actually experiences.
Low density brings its own dangers for other species. When a population thins out, finding a mate gets harder and fertility falls, which can spiral toward extinction. Ecologists call this the Allee effect, after the scientist who described it.
Source: Population density
