Filter strips are installed along the margins of agricultural fields to trap sediment and other pollutants conveyed in overland runoff. A geographical information system (GIS) procedure is presented for designing a filter strip that will achieve a constant, user-selected level of trapping efficiency along a field margin. The design model is based on relationships between the buffer area ratio and trapping efficiency, which account for pollutant type and site conditions. The design model has been coupled with GIS and terrain analysis in a computer program called AgBufferBuilder to run with ArcGIS for quickly producing filter strip designs that can vary in width along a field margin to compensate for concentrated runoff flows. The design procedure is demonstrated on a sample agricultural field. The computer program, user’s guide, and demonstration datasets can be downloaded from the website https://www2 .ca.uky.edu/BufferBuilder.