The interface between wetting (W) and non-wetting (NW) fluids in a porous medium includes contributions from three morphologies. One is the interface between the bulk connected volumes of W and NW phases. A second arises when blobs of one phase become disconnected from the parent bulk phase. Examples of this morphology include pendular rings of W phase held at grain contacts and lenses of W in pore throats. Finally, there is the thin film of W coating the walls of pores containing NW. We describe a theoretical means of estimating these contributions during drainage (NW displacing W). The technique is based on a physically representative and geometrically determinate network model of granular media that allows a priori predictions of pore-scale transport phenomena. The predicted total from all three wetting phase morphologies agrees with the trend of measurements reported in the literature, indicating that the experimental techniques used did not discriminate between the morphologies.