The report describes a digital computer program for estimating the capital cost, the amortization cost, and the operating and maintenance cost for conventional wastewater treatment plants made up of subsets of current processes. The design relationships used in the program for sizing the individual processes were made as simple as possible. Engineering relationships were used to compute the quantity of sludge produced, the oxygen requirements, etc. This program can be used to estimate the cost of primary sedimentation plants, activated sludge plants, and trickling filter plants. Cost estimates generated by the program for various sizes of plants are shown in the third section of this report. The program is written in FORTRAN and is designed to run on a 16K IBM 1130 computer.