The Rank-Size Rule and Primate Cities

The rank-size rule is a pattern, not a law: in many countries with well-developed urban systems, the second-largest city holds roughly half the population of the largest, the third holds roughly a third, the fourth roughly a quarter, and so on. The formula is simple: predicted population of the city ranked n equals the largest city's population divided by n. This pattern shows up because a mature urban hierarchy distributes economic functions and population across many cities of varying sizes, from the massive capital down through regional hubs to small towns, each filling a niche sized to its rank.

Not every country follows this. When one city vastly outranks every other, usually holding more than twice the population of the second-largest and overshadowing the rest of the country in politics, culture, and economy, geographers call it a primate city. Bangkok, Thailand's capital, dwarfs Thailand's second city by a huge margin and concentrates the country's government, media, and migration pull in one place. Size alone doesn't make a city primate; a city that is merely the biggest by a small margin, fitting neatly into a rank-size pattern, is not primate. The gap and the dominance are what matter.

This topic connects to the other models in the unit because it describes the city system of a whole country, while Burgess, Hoyt, and the multiple nuclei models describe the inside of one city, and Christaller's central place theory explains why cities of different sizes are spaced the way they are. Primacy often signals an uneven, colonial, or highly centralized history, where one city absorbed national investment while others were left behind.