Human population density maps, such as the one in SF Fig. 2.1.1, are maps showing the number of people in an area. In 2011 there were approximately seven billion people in the world. Earth’s total land area is approximately 150 million square kilometers (km²).

If every human on the planet in 2011 were placed into one area, people would take up a relatively small amount of space. For example, if everyone were given 0.9 square meters (m²), which is about the area of made by one arm span, the entire world’s population would take up 6,300 km2 of space. So, with only room enough for each person to spin around with their arms extended, the entire human population could mathematically fit in an area the size of the largest island in the Hawaiian island chain, the Island of Hawai‘i (10,430 km²)!

When a number of people is placed in a smaller area, human density is increased. Density can also increase by adding more people to a given area. In other words, when the total human population increases, human density on the planet also increases because the amount of space available for humans to live on earth is relatively constant.