Glossary

Gravity Model (Reilly's Law of Retail Gravitation)

Also known as: Reilly's Law of Retail Gravitation, retail gravitation model

A location-theory model, originating with Reilly's Law of Retail Gravitation, that predicts the geographic breakpoint between two competing commercial centers based on their relative size (drawing power) and the distance between them — analogous to gravitational attraction, where larger mass and shorter distance both increase pull.

Reilly's original formulation calculates the breakpoint — the distance from the smaller center at which consumers become indifferent between traveling to it or to the larger, more distant competitor — as a function of the square of the distance between the two centers and the ratio of their relative sizes (typically measured in retail square footage or sales volume), directly borrowing the inverse-square relationship from Newtonian gravitational physics. The model's core insight, that drawing power increases with size but decays with distance, remains foundational to trade area delineation and competitive impact analysis even though the original deterministic breakpoint formulation has largely been superseded in practice by probabilistic successors like the Huff Model, which better reflect that real consumers do not switch allegiance at a hard line but instead patronize competing centers with gradually shifting probability as distance and relative attractiveness change. Gravity-model logic underlies most modern retail and mixed-use site-selection software, even where the specific mathematical implementation has evolved well beyond Reilly's original 1931 formula.