A step‐by‐step design approach for sizing anoxic/oxic single‐sludge activated sludge systems is presented. A computer model utilizing an EXCEL spreadsheet was also developed which simulates the pre‐denitrification single‐sludge activated sludge process. The biokinetic equations developed by Lawrence and McCarty are utilized in the model. The model was calibrated using data from a full‐scale, pre‐denitrification activated sludge process. Two‐tailed, paired comparison Student's “t‐test”; performed on effluent BOD5, SS, TKN, NH3‐N, NO3‐‐N, and TN indicated there was no significant difference between the actual effluent concentrations and those predicted by the model at a 95 percent confidence level.