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	Reilly's Law of Retail Gravitation	Gravity Model of Migration
Field	Human Geography	Human Geography
Family	Regression model	Regression model
Year of origin	1931	1946
Originator	William J. Reilly	George Kingsley Zipf (formalized); analogy to Newton's law of gravitation
Type	Deterministic gravity model of retail trade-area delineation	Spatial-interaction regression model for migration flows
Seminal source	Reilly, W. J. (1931). The Law of Retail Gravitation. Knickerbocker Press, New York.	Zipf, G. K. (1946). The P1 P2 / D hypothesis: On the intercity movement of persons. American Sociological Review, 11(6), 677–686.
Aliases	Law of Retail Gravitation, Reilly's Retail Gravitation Model, Retail Breaking-Point Model, Reilly Gravity Model	Migration Gravity Model, Demographic Gravity Model, Zipf P1P2/D Model, Gravity Model of Spatial Interaction (Migration)
Related	4	4
Summary	Reilly's law of retail gravitation is a deterministic model that predicts how an intermediate town's retail trade divides between two larger competing cities. Formulated by William J. Reilly in 1931 by analogy with Newtonian gravity, it states that each city attracts trade in direct proportion to its population and in inverse proportion to the square of the distance to it. Solving for the point of equal attraction yields the famous breaking point — the boundary along the route between two cities where their trade areas meet.	The gravity model of migration explains the volume of movement between two places as proportional to the product of their populations (masses) and inversely proportional to the distance separating them, by direct analogy to Newton's law of universal gravitation. Formalized for intercity movement by George Kingsley Zipf in 1946 and embedded in regional science by Walter Isard, it is the workhorse model of human geography for predicting migration, commuting, and other spatial-interaction flows.