GLP-1 Weight Loss Calculator

How the predictions are calculated

Most online weight-loss calculators use simplistic percentages (“you'll lose 10-15% on a GLP-1”). This calculator does something different: it takes the actual published week-by-week mean weight loss curves from three phase 3 trials and scales them to your starting weight using linear interpolation between the published datapoints.

The three trials powering the calculator:

• STEP-1 [1] — Wilding et al., NEJM 2021, n=1,961, semaglutide 2.4 mg weekly, 68 weeks. Final endpoint: −14.9%.
• SURMOUNT-1 [2] — Jastreboff et al., NEJM 2022, n=2,539, tirzepatide 15 mg weekly, 72 weeks. Final endpoint: −20.9%.
• ATTAIN-1 [3] — Eli Lilly 2026 FDA approval trial, n=3,127, orforglipron (Foundayo) 17.2 mg daily oral, 72 weeks. Final endpoint: −12.4%.

At each published week in the trial (weeks 4, 8, 12, 16, 20, 28, 40, 52, and the final endpoint), the calculator uses the reported mean body weight loss as a percentage. Between those datapoints, a linear interpolation approximates the curve. Your predicted absolute weight loss at any week is simply (starting weight × mean loss %).

What the conservative and optimistic ranges mean

The low and high estimates in the prediction card are approximately ±25% of the mean, which encompasses roughly the interquartile band of the STEP-1 patient-level variability reported in the trial supplementary data. In STEP-1, about 86% of participants achieved at least 5% weight loss on semaglutide [1] — so the “low” estimate corresponds to approximately the 25th percentile responder, the mean to the 50th percentile, and the “high” to approximately the 75th percentile. Patients in the top 10% of responders often lose considerably more than the “optimistic” estimate suggests; patients in the bottom 10% may lose considerably less.

What the calculator doesn't account for

This is an educational tool that approximates the published trial data. It does not account for:

• Individual variation. Starting BMI, age, sex, baseline metabolic rate, diet quality, physical activity, and sleep all affect response. Trial means hide significant patient-level variance.
• Dose-response at lower doses. The predictions assume you are progressing through the standard titration schedule and reaching the maintenance dose that the trial used. Patients who stay at a lower dose indefinitely (microdosing) will see less weight loss — see our microdose evidence guide for the dose-response data.
• Injection technique errors. Repeated injection into a lipohypertrophic site can reduce absorption by 25-50% and cause an apparent “non- response” that is actually an injection issue. See our injection guide.
• Compounded dose math errors. If you are on compounded semaglutide or tirzepatide and the unit-to- mg conversion is wrong, your effective dose can be half or double the intended. See our unit converter.
• Protein intake and exercise. The STEP and SURMOUNT trials all included dietary counseling. Patients who ignore the dietary side of therapy generally do worse than the trial average — see our diet guide and muscle mass deep-dive.
• Discontinuation rebound. The STEP-4 extension trial showed that patients who stopped semaglutide regained roughly 67% of the lost weight within a year. Predictions assume continued treatment.

Important disclaimer

This tool is for educational purposes only and does not constitute medical advice. The predicted values are scaled from the published mean weight loss curves of phase 3 clinical trials and should not be interpreted as a guarantee of your personal outcome. Individual results vary substantially based on factors the calculator cannot account for. Weight Loss Rankings does not provide medical advice, diagnosis, or treatment recommendations.

Related tools and research

• GLP-1 dose plotter — the plasma concentration buildup curves behind these weight loss numbers
• GLP-1 unit converter — mg ↔ insulin syringe units for compounded dosing
• How long does GLP-1 take to work? — the three separate timescales explained
• Tirzepatide vs semaglutide head-to-head — why SURMOUNT-1 produces larger weight loss than STEP-1
• What happens when you stop semaglutide — the STEP-4 extension data on rebound