Economic potential measures of accessibility seem to have been rediscovered in the research literature recently, as well as in research that informs policy formulation. These new applications are using more and more sophisticated sources of data but are in large measure still operationalising the familiar concepts of market potential. Such potential is calculated for any zone by summing the representative economic mass of all other zones in the system each divided by some measure of the intervening travel impedance between that zone and every other zone. In this straightforward calculation it becomes necessary to incorporate the economic mass of the zone under consideration itself and to decide on the appropriate travel impedance. This apparently simple task is the focus of this paper. Most research of this type uses a weighting of the radius of the circle equalling the area of the zone in question to approximate the travel impedance. The purpose of this paper is to demonstrate that the choice of weighting is important in determining the nature of the resultant potential surface.