This article discusses a study conducted at Iowa State University to develop a method of analyzing data from pilot tests with a raw water supply for use in the design of a diatomite filter plant. A method was developed for obtaining plant operating conditions at a minimum cost by the specification of the overall optimum body feed, the optimum filtration rate, and the optimum terminal‐head loss that will permit water to be filtered at the lowest total cost per 1,000 gal. A method of finding this optimum design combination is demonstrated for three cities of different sizes with waters of three different iron concentrations.