The Walker Pond Size Model is a widely used spreadsheet approach to the design of storm water treatment ponds. The user inputs the Soil Conservation Service curve number for the pervious areas and the fraction of impervious area. Input parameters must be correctly determined by the user. One commonly made error is to enter the weighted curve number in place of the pervious curve number, which can result in overestimation of dead storage volume by as much as 8.89 mm (0.35 inch). Using the weighted curve number only and not the model's equation can underestimate the required volume as much as 19.56 mm (0.77 inch). Proper use of the Walker Model is essential to obtain a beneficial, cost-effective design.