The primacy index (or index of primacy) is a measure of how much the largest city in a country or region dominates the urban system in terms of population size.

Basic idea

The index shows whether the urban system is “primate” (one city overwhelmingly larger than others) or more balanced (several large cities of similar size). A higher primacy index implies greater concentration of population and resources in the first‑rank city, which often reflects strong centralization in politics, economy, and services.

Common formulas

1. Two‑city primacy indextext\text{PI} = \frac{P_1}{P_2}where P_1 is the population of the largest city and P_2 is the population of the second largest.
   • If PI is greater than 2, the city is often considered “primate” (Mark Jefferson’s rule).
2. Three‑city primacy index PI = \frac{C_1}{C_1 + C_2 + C_3} \times 100where C_1, C_2, C_3 are the populations of the first, second, and third largest cities.​

Interpretation in planning

• High primacy index: indicates strong centripetal forces, agglomeration bias, and often regional imbalance.
• Low or declining index: suggests a more polycentric or balanced urban system, which many planners view as more sustainable.