An integrated circuit design tool for modeling resistance of an interconnect specifies a structure of the interconnect in a data structure in memory in or accessible by the computer system using 3D coordinate system. For each of a plurality of volume elements in the specified structure, the tool specifies a location and one of first and second materials of the interconnect having specified resistivities, and for each volume element generates a model resistivity for the volume element as a function of resistivity of volume elements within a neighborhood of the volume element and a specified transition region length λ.