Population density — the number of people per unit of land area — is one of the most variable demographic measures in the world. It ranges from over 26,000 people per square kilometer in Monaco to just 2 in Mongolia, a difference of more than 13,000-fold.

Several factors drive these differences. Physical geography is the most fundamental: people cluster near coastlines, rivers, and in temperate climates. About 40% of the world's population lives within 100 kilometers of a coast. Mountain ranges, deserts, and frozen regions naturally limit settlement.

Bangladesh is a striking example: it has over 1,700 people per km² because it sits on a fertile river delta with abundant water and agriculture, despite being prone to flooding. The Netherlands, at over 500 per km², has engineered its low-lying land to support dense settlement.

At the other extreme, countries like Mongolia, Australia, and Canada have vast land areas but harsh or dry interiors that keep most people concentrated in a few regions. Over 80% of Australians live within 50 kilometers of the coast.

Density matters for policy. High-density areas face challenges in housing, traffic, and resource management, but they also benefit from agglomeration effects — economies of scale in infrastructure, services, and innovation. Low-density countries face different challenges: delivering healthcare, education, and connectivity to scattered populations.