GLP-1 Weight Loss Calculator

How the predictions are calculated

The usual approach online is a single percentage — you will lose 10 to 15% on a GLP-1 — which is true of nobody in particular. What happens here instead is that the week-by-week mean curves three phase 3 trials actually published are scaled to your starting weight, interpolating linearly between their datapoints.

The three trials powering the calculator:

• STEP-1 [1] — Wilding and colleagues in the NEJM, 2021. 1,961 participants, semaglutide 2.4 mg weekly, 68 weeks, finishing at −14.9%.
• SURMOUNT-1 [2] — Jastreboff and colleagues in the NEJM, 2022. 2,539 participants, tirzepatide 15 mg weekly, 72 weeks, finishing at −20.9%.
• ATTAIN-1 [3] — the Lilly trial behind the 2026 FDA approval. 3,127 participants, orforglipron (Foundayo) 17.2 mg as a daily tablet, 72 weeks, finishing at −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 cautious and optimistic figures sit about 25% either side of the mean, which is roughly the interquartile band STEP-1's supplementary data reported for individual variation. Around 86% of its participants reached at least 5% loss on semaglutide [1]. So read the low figure as approximately the 25th-percentile responder, the mean as the 50th, and the high as the 75th — and remember the tails run past both. The top tenth commonly beat the optimistic figure by some margin; the bottom tenth fall well short of the cautious one.

What the calculator doesn't account for

This approximates published trial data and no more. Things it knows nothing about:

• Individual variation. Where your BMI starts, your age, your sex, how fast you burn energy at rest, what you eat, how much you move, how you sleep — all of it moves the result, and a trial mean conceals how widely individuals scattered around it.
• 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.
• Getting the compounded arithmetic wrong. On a compounded semaglutide or tirzepatide, a mistake converting units to milligrams does not produce a small error — it halves or doubles what you actually take. Our unit converter.
• Discontinuation rebound. In the STEP-4 extension, people who came off semaglutide had put back about two-thirds of what they lost — roughly 67% — inside a year. Everything projected here assumes you stay on the drug.

Important disclaimer

Educational only, and not medical advice. Every number projected here is a published phase 3 mean scaled to your weight, which makes it an expectation rather than a promise. What any one person gets swings a long way on things this page cannot see. GLP Watchdog does not diagnose, does not treat, and does not tell you what to take.

Related tools and research

• GLP-1 dose plotter — how the drug itself accumulates behind those weight loss numbers
• GLP-1 unit converter — milligrams to syringe units, for a compounded vial
• How long does GLP-1 take to work? — the three separate timescales explained
• Tirzepatide vs semaglutide head-to-head — what puts SURMOUNT-1 ahead of STEP-1
• What happens when you stop semaglutide — the STEP-4 extension data on rebound