Some places feel crowded; others feel empty. Population density measures the number of people per unit area of land, usually expressed as persons per square kilometer or square mile. It reveals how intensively a territory is occupied.

High densities occur in fertile river valleys, coastal plains, and urban centers. Low densities characterize deserts, high mountains, and boreal forests. Arithmetic density simply divides total population by total area; physiological density relates population to arable land and often gives a more meaningful picture of pressure on resources.

Density influences the cost of providing services, the intensity of land use, and the potential for both innovation and congestion. Maps of population density are among the most revealing images of human geography. They show at a glance where people have chosen—or been constrained—to live.

• People per unit area of land
• Highlights uneven human distribution
• Higher in cities and fertile regions
• Important for planning and resource assessment