A layered streamtube model for estimating waterflood performance has been developed. The model is suitable for microcomputer applications. Displacement mechanisms are represented with a linear Buckley-Leverett11 calculation including, if desired, the presence of a free gas saturation or mobile water saturation at the start of flooding. Areal sweep effects are included by using fixed streamtubes for various patterns. By considering production and pressure drop to be a function of cumulative injection, rather than distance of the flood front along a streamtube, the necessity for geometric factors is greatly reduced. Reservoir heterogeneity and vertical sweep effects are considered with the Dykstra-Parsons approach. Pseudo-relative permeabilities can be used to account for crossflow with the Hiatt-Hearn model4,5 for uniform vertical pressure. Using the proposed model, the factors having the greatest effect on performance can be determined. Buckley-Leverett effects, including production after water breakthrough and variations in mobility ratio, can be as significant as reservoir heterogeneity. The presence of free gas at the start of water injection is shown to have a significant impact.