The Revised National Plumbing Code of the Philippines of 1999 requires the use of and construction of septic tank based from the provisions and specifications provided in the code. However, many septic tanks are incorrectly designed and improperly constructed resulting to environmental contamination. Hence, this study developed a spreadsheet program for designing septic tank for residential dwelling units that would aid designers, engineers and master plumbers. The program was developed using MS Excel ® (Office Home and Student 2010 – Family Pack) as it is the most popular and widely used spreadsheet software. The program was evaluated by comparing its output with manual solution. The results indicated that the spreadsheet program can be developed to hasten the design of septic tank. It also provides sketch plans and bill of materials for the said plumbing structure. However, it is recommended to develop other septic design computer programs for apartment and other types of occupancy.