A low-cost alternative for treating wastewater generated in rural and urban areas are septic tanks. The removal efficiency of pollutants depends on septic tank effluent characteristics and particularly on the size of the tank. The objective of this work was to develop a tool application that calculates the size of a rectangular septic tank for domestic or public service with a maximum volume of about 15 000 L. The FOSEP application, built with Visual Basic programming language v. 6.0, calculates the sizing variables length (L), width (A), minor tie bar (H1), major tie bar (H2) and total volume (Vt). The FOSEP tool was applied to a case study to evaluate the design and operation of a septic tank treating the domestic wastewater generated by 60 users of "The Alberto" community in the state of Hidalgo. The design analysis carried out by means of the FOSEP tool showed that the septic tank was oversized, but it removes efficiently COD, TSS, VSS and fecal coliforms.