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Weight-loss projection from trial averages

Apply the published mean percent weight change from STEP 1 (semaglutide 2.4 mg) and SURMOUNT-1 (tirzepatide 5, 10 and 15 mg) to your own weight, with the placebo arm alongside. Population averages, clearly labelled, with the model shown.

By FormBlends editorial teamAssumptions checked September 4, 2026Runs in your browser; nothing is sent to a server

Inputs

lb

Each arm is a randomised group in a published trial. The number applied is that group's mean percent change at the trial's final week.

STEP 1

Wilding JPH et al. Once-weekly semaglutide in adults with overweight or obesity. N Engl J Med 2021;384:989-1002.

PubMed 33567185. Treatment-policy estimand (all randomised participants regardless of adherence). Mean baseline weight 105.3 kg.

Population-average projection

Enter a weight. The projection applies the trial arm's average percent change to it, week by week.

Formulashown in full, nothing hidden
fraction(t)   = (1 - exp(-k t)) / (1 - exp(-k T))     for 0 <= t <= T,  k = 0.04 per week
percent(t)    = endpoint percent change x fraction(t)
weight(t)     = start weight x (1 + percent(t) / 100)
T             = trial endpoint week (68 for STEP 1, 72 for SURMOUNT-1)

The endpoint percent change is the published primary result for the arm. The curve shape between week 0 and T is a first-order approach model, not digitised trial data; k was chosen so that about 60 percent of the total change is reached by week 20, which matches the shape of the published figures. After week T the projection stops, because the papers report nothing beyond it.

Assumptions, checked September 4, 2026

  • STEP 1 (PubMed 33567185): semaglutide 2.4 mg weekly for 68 weeks, mean change in body weight minus 14.9 percent versus minus 2.4 percent with placebo, treatment-policy estimand; adults with BMI 30 or more, or 27 or more with at least one weight-related condition, without diabetes.
  • SURMOUNT-1 (PubMed 35658024): tirzepatide 5, 10 and 15 mg weekly for 72 weeks, mean change minus 15.0, minus 19.5 and minus 20.9 percent versus minus 3.1 percent with placebo, treatment-regimen estimand; similar population without diabetes.
  • Both trials included lifestyle counselling (a reduced-calorie diet and increased physical activity) in every arm, including placebo.
  • Population means are not individual predictions. Trial participants were selected, monitored and adherent in ways that routine care may not reproduce.
  • Results for compounded products have not been studied in these trials, and compounded semaglutide and tirzepatide are not FDA approved.

This calculator is an arithmetic aid for medication a licensed prescriber has already prescribed. It does not recommend a dose. Confirm any result with your prescriber or pharmacist before use.

Sources

  1. Wilding JPH et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). NEJM 2021. PubMed 33567185
  2. Jastreboff AM et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). NEJM 2022. PubMed 35658024

Read before you use it

Guides for this calculator

What a population average means: reading the weight-loss projection

Where the STEP 1 and SURMOUNT-1 numbers come from, what the curve between week 0 and the endpoint is and is not, and the three reasons your own result will differ from the line.

Updated September 4, 2026

Educational, not medical advice. Read the medical disclaimer.