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Highlights • Weight-loss assessment after bariatric-surgery should become more accurate, fair and evidence-based. • Multicenter observational study, 9393 laparoscopic Roux-en-Y gastric bypass and laparoscopic sleeve gastrectomy patients from three major bariatric clinics. • Weight-loss percentile-charts (similar to WHO growth-charts for children) are presented for the two worldwide most popular bariatric procedures. • These charts help assess bariatric weight-loss, weight-regain and poor responders at any postoperative time up to 7-years, regardless of the preoperative body-mass-index. They could form the ideal base for bariatric e-health applications. Abstract Background Current methods for weight loss assessment after bariatric surgery do not meet the high standards required to accurately judge patient outcome in a fair and evidence-based way. Objectives To build an evidence-based, versatile tool to assess weight loss and weight regain and identify poor responders up to 7 years after laparoscopic Roux-en-Y gastric bypass (LRYGB) and laparoscopic sleeve gastrectomy (LSG), for any preoperative body mass index (BMI). Setting Multicenter, observational study. Methods Bariatric weight loss charts were built with standard deviation (SD) percentile (p) curves p+2SD/p+1SD/p50(median)/p−1SD/p−2SD, based on all last measured weight results after primary LRYGB and LSG, performed in 3 large bariatric centers, expressed with percentage total weight loss (%TWL) and percentage-alterable weight loss (%AWL), a special BMI-independent metric. The p−1SD %AWL curves were compared with popular bariatric criteria 50% excess weight loss and 20%TWL. The p50 %TWL curves were compared with %TWL outcome in literature (external validation). Results In total, 9393 patients (5516 LRYGB, 3877 LSG, baseline BMI 43.7 (±SD 5.3) kg/m2, age 43 (±SD 10.9) years, 20% male, 21% type 2 diabetes) had mean follow-up 26 (range, 0–109) months, with.09% 30-day mortality. Independent outcome is presented in percentile charts for %AWL and %TWL. Percentile curves p+2SD/p+1SD/p50/p−1SD/p−2SD showed for LRYGB 72%/62%/50%/38%/28%AWL at nadir, 66%/55%/43%/30%/17%AWL at 4 years, 64%/52%/38%/25%/11%AWL at 7 years, and for LSG 69%/58%/46%/34%/22%AWL, 65%/53%/38%/23%/12%AWL, and 63%/51%/35%/22%/9%AWL, respectively. Bariatric criteria 50% excess weight loss and 20%TWL matched with most insufficient results for LSG, but not for LRYGB (low specificities). Both p50 %TWL curves are comparable with long-term weight loss in bariatric literature. Conclusions Just as well-known growth charts are essential for pediatrics, weight loss charts should become the tools of choice for bariatrics. These multicenter charts are baseline BMI independent, superior to current bariatric criteria, and quite intuitive to use. They allow to readily detect poor responders in any postoperative phase, monitor the effect of extra counseling, judge weight regain, and manage patient expectations. [ABSTRACT FROM AUTHOR]