GLP-1 Weight Loss Calculator
The trials report percentages. This turns them into kilograms for your starting weight, and separates the part that was fat from the part that was not. It does not calculate a dose — GLP-1 dosing is set by a prescriber, not by a website.
Enter your starting weight and the medication.
Why this is not a dose calculator
GLP-1 medications are titrated — started low and raised on a schedule, watching tolerance. Semaglutide runs 0.25 mg to 2.4 mg over months; tirzepatide runs 2.5 mg to 15 mg. The schedule depends on side effects, other medication and clinical judgement, none of which a calculator can see.
What a calculator can do is answer the question the trials leave hanging: the papers report percentages, and nobody thinks in percentages about their own body.
Why the lean mass number is not a percentage
The weight-loss figures are percentages of starting weight, so they scale — 14.9% of 90 kg is more kilograms than 14.9% of 70 kg, and the trial supports that.
The lean mass figure does not work that way. In SEMALEAN, a cohort of 106 people on semaglutide 2.4 mg, lean mass fell by about 3 kg by month 7 and then stabilised rather than continuing down. That is an absolute figure from one cohort, not a rate that scales with bodyweight. Turning it into a percentage would be inventing a number the study never produced, so this page reports it as what it is.
The same cohort is worth reading in full: fat mass fell 18% by month 12, and grip strength — the standard functional measure — was 4.5 kg higher at 12 months than at baseline. Lean mass down, measured strength up. The full breakdown is here.
Questions
Will I actually lose this much?
The number is an average, and averages hide a wide spread. In STEP-1, 86% of participants lost at least 5% of body weight — which also means 13.6% did not reach 5%. Some lost far more than 14.9%, some lost almost nothing. Treat the figure as the middle of a distribution, not a promise.
Why is tirzepatide higher?
SURMOUNT-1 reported 16.0% to 22.5% depending on dose over 72 weeks, against STEP-1's 14.9% for semaglutide over 68. The trials were run separately with different populations and lengths, so this is not a head-to-head. The one direct comparison, SURMOUNT-5, put tirzepatide at 21.5% against semaglutide's 14.5%.
What happens if I stop?
Weight regain after stopping is well documented, and it is the part least discussed when the medication is started. What the follow-up data shows.
Does the lean mass loss matter?
Some lean mass loss accompanies any large weight loss, drug or not. What the evidence supports doing about it is resistance training and protein intake — roughly 1.6–2.2 g per kg of bodyweight per day, which is genuinely hard when appetite is suppressed. The protein calculator sets the target.