Objective: To use clinician reported data pre- and post-operative in bariatric surgery patients to create an algorithm for the development of a standardized tool, for use by bariatric surgeons within a confidence interval, to determine the maximum weight loss threshold from roux en y surgery by the third physician visit. Methods: Retrospective database analysis (2000-2007) of empirical clinical data, pre- and post-operative, for bariatric patients in Western New York. A multivariate model examined the relationship between % excess body weight lost (BWL) at the first three post-operative visits and % BWL at the sixth post-operative visit (V6), using SAS 9.1. Percent excess BWL was plotted vs. days elapsed from surgery. 179 obese adults (women=155, 86.6%) received gastric bypass surgery, mean BMI=53, (SD=9.6), mean excess body weight at time of procedure =185 lbs (SD=65.3). Results: Outcomes were available for 158 patients (women=137, 86.7%) at V6 (mean=707 days), mean BMI=35.2 (SD=7.7), women=34.7 vs. men=38.1. Mean % excess BWL at V6 was 60% (SD=18%). Women had more BWL than men (61% vs. 55%). For females, BWL was maximized at 870 days post-operation, the equation fit for males did not yield an absolute maximum, leveling off at 740 days (monthly change rate<0.5%/month). Our model included linear and non-linear components to correlate the relationship between the total % excess BWL at V6 and % excess BWL at the first three visits. All variables, except BWL at the second visit, were statistically significant. The algorithm had a predictive accuracy of 94%. Conclusion: The excess BWL at the third visit is predictive of the final excess BWL, following surgery. Gender and initial BMI displayed significant relationships with final weight loss. The algorithm derived from this sample will support the development of a standardized tool to assist physicians in their post-operative prognosis of gastric bypass patients.