Spatially nonuniform runoff reduces the water quality performance of constant-width filter strips. A geographic information system (GIS)-based tool was developed and tested that employs terrain analysis to account for spatially nonuniform runoff and produce more effective filter strip designs. The computer program, AgBufferBuilder, runs with ArcGIS versions 10.0 and 10.1 (Esri, Redlands, California) and uses digital elevation models to identify detailed spatial patterns of overland runoff to field margins. The tool then sizes filter dimensions according to those patterns using buffer area ratio relationships. The resulting design is larger along segments where more runoff flows and smaller along segments where runoff is less and delivers a constant level of trapping efficiency around the field margin for sediment and sediment-bound pollutants. The tool also can estimate trapping efficiency of existing filter strips or hypothetical configurations. In a validation test, estimates of sediment trapping efficiency using the tool's assessment function compared closely to measurements taken on large field plots in central Iowa. Using AgBufferBuilder, designs developed for a sample of fields in the midwestern United States were estimated to trap nearly double the sediment, on average, during a design storm than constant-width configurations having equivalent total filter area. AgBufferBuilder can be used to bolster environmental performance of filter strips where runoff is spatially nonuniform. The AgBufferBuilder tool is publicly available on the websites http://www2.ca.uky.edu/BufferBuilder and http://nac.unl.edu/tools/AgBufferBuilder.