The rank-size rule is a simple pattern that describes how city sizes are distributed within a country. It says that the population of a city is inversely proportional to its rank in the urban hierarchy.

Quick Scoop

  • If you rank cities from largest (rank 1) to smaller (rank 2, 3, 4, …), each city’s size is about the largest city’s size divided by its rank.
  • In formula form:

P(r)≈P1rP(r)\approx \frac{P_1}{r}P(r)≈rP1​​

where P(r)P(r)P(r) is the population of the city with rank rrr, and P1P_1P1​ is the population of the largest city.

  • So:
    • 2nd-largest city ≈ 1/2 the population of the largest.
    • 3rd-largest ≈ 1/3 of the largest.
    • 10th-largest ≈ 1/10 of the largest.

This rule is widely used in urban geography to understand how evenly population and economic functions are spread across a country’s cities: when the rule holds, the system is relatively balanced ; when it doesn’t, one primate city (like an oversized capital) may dominate.

Tiny example

Imagine the largest city has 10 million people:

  • Rank 1: 10 million (largest city).
  • Rank 2: ≈ 5 million.
  • Rank 3: ≈ 3.3 million.
  • Rank 4: ≈ 2.5 million.

All of these are just approximations, but they follow the rank-size rule pattern. TL;DR: The rank-size rule says the nnnth-largest city in a country tends to have about 1/n1/n1/n of the population of the largest city, giving a predictable hierarchy of city sizes. Information gathered from public forums or data available on the internet and portrayed here.