GLP-1 Weight Loss Projection
The two biggest GLP-1 trials gave us clean month-by-month weight loss curves. Knowing what the average user lost on each drug — and at what dose — is the difference between feeling like a failure and recognizing you are exactly on track.
The honest answer to "how much weight will I lose on a GLP-1?" is: it depends on the drug, the dose, your starting weight, and roughly six months of consistent dosing before the curve starts to plateau. The averages from the pivotal trials give you a defensible projection, not a guarantee.
This page projects from the two trials that anchor modern GLP-1 prescribing: STEP 1 for semaglutide 2.4mg (Wegovy) and SURMOUNT-1 for tirzepatide 15mg (Zepbound). Both ran for 68-72 weeks and reported intent-to-treat results, which means they included people who quit early. Your projection, if you stay on the medication consistently, is closer to the completer-analysis numbers, which are 2-4 percentage points higher.
Real weight loss does not follow a smooth line. It goes hard for the first 8-12 weeks, slows around weeks 16-24, accelerates again with dose escalations, and finally plateaus somewhere between months 9 and 14. The numbers below are averages — your line will be jagged.
The formula
The projection is simple: take your starting weight, multiply by the percentage loss expected at each month milestone for your drug, and that gives you the expected weight at that point. There is no fancy curve fitting needed — the trial percentages do the work.
The two drug curves are different shapes. Semaglutide builds slowly and lands around 14.9% total body weight loss at month 12. Tirzepatide ramps faster and lands around 20% at month 12 (and 22.5% for the maximal-completer subgroup in SURMOUNT-1). The gap is not trivial — for a 240-pound starting weight, that is the difference between 35.8 lb and 48 lb at one year.
The Formula
Expected weight (lb) = starting weight × (1 − loss percentage)
Sema (2.4mg): 6% at mo 3, 10% at mo 6, 13% at mo 9, 14.9% at mo 12. Tirz (15mg): 10% at mo 3, 15% at mo 6, 18% at mo 9, 20% at mo 12.
Worked examples
Person A: 240 lb starting, semaglutide 2.4mg
Month 3: 240 × 0.94 = 226 lb. Month 6: 240 × 0.90 = 216 lb. Month 12: 240 × 0.851 = 204 lb (35.8 lb total loss).
Person B: 240 lb starting, tirzepatide 15mg
Month 3: 240 × 0.90 = 216 lb. Month 6: 240 × 0.85 = 204 lb. Month 12: 240 × 0.80 = 192 lb (48 lb total loss).
Person C: 180 lb starting, semaglutide 2.4mg
Month 6: 180 × 0.90 = 162 lb. Month 12: 180 × 0.851 = 153 lb (27 lb total loss). Lower-starting-weight users tend to land at the lower end of the trial range.
These projections assume you reach the maintenance dose (2.4mg sema or 10-15mg tirz) and stay there. If you stall on a lower dose, the curve flattens earlier. If you stop the medication, the published rebound data suggests two-thirds of lost weight returns within a year for semaglutide — the curve does not stay flat without the drug.
Where the calculator will live
An interactive version — where you enter starting weight, choose a drug, and see your projected curve plotted against the trial average with a confidence band — is on the roadmap. Until then, the percentages above let you do the same calculation on a napkin. For a more conservative projection that adjusts for sub-trial real-world response rates, drop each number by 2-3 percentage points.
Common questions
Common Concerns
Why is tirzepatide so much more effective?expand_more
What if my weight loss is slower than the projection?expand_more
Are these projections lifetime totals?expand_more
Does starting BMI change the projection?expand_more
Keep exploring
Browse all dosing guides.