Background Currently, there is no agreement on the best method to describe weight loss (WL) after bariatric surgery. The aim of this study is to evaluate short-term outcomes using percent of total body weight loss (%TWL). Methods A single-institution retrospective study of 2420 patients undergoing Roux-en-Y gastric bypass (RYGB) was performed. Suboptimal WL was defined as %TWL < 20 % at 12 months. Results Mean preoperative BMI was 46.8 ± 7.8 kg/m 2 . One year after surgery, patients lost an average 14.1 kg/m 2 units of body mass index (BMI), 30.0 ± 8.5 %TWL, and 68.5 ± 22.9 %EWL. At 6 and 12 months after RYGB, mean BMI and percent excess WL (%EWL) significantly improved for all baseline BMI groups ( p  < 0.01, BMI; p  = 0.01, %EWL), whereas mean %TWL was not significantly different among baseline BMI groups ( p  = 0.9). The regression analysis between each metric outcome and preoperative BMI demonstrated that preoperative BMI did not significantly correlate with %TWL at 1 year ( r  = 0.04, p  = 0.3). On the contrary, preoperative BMI was strongly but negatively associated with the %EWL ( r  = −0.52, p  < 0.01) and positively associated with the BMI units lost at 1 year ( r  = 0.56, p  < 0.01). In total, 11.3 % of subjects achieved <20 %TWL at 12 months and were considered as suboptimal WL patients. Conclusion The results of our study confirm that %TWL should be the metric of choice when reporting WL because it is less influenced by preoperative BMI. Eleven percent of patients failed to achieve successful WL during the in the first year after RYGB based on our definition.