GLP-1 Weight Loss Calculator: 12-Month Projection From the Trials

1. Choose the dose you expect to stay on: tirzepatide 5, 10 or 15 mg, the three SURMOUNT-1 arms, or semaglutide 2.4 mg from STEP 1 or 7.2 mg from its 72-week trial.
2. Enter your starting weight in pounds, kilograms or stones.
3. If you have started, enter the date and your weight today to see where you sit against the trial curve and its placebo line at your week. If not, add a date that matters to read your weight on it; before you start, the three tirzepatide doses show as a range.
4. Read the weight at your date and at month 12, and, if you add a goal, the month the average curve reaches it.

The result is the trial average scaled to your weight, not a forecast for you. People on a lower dose, and people with type 2 diabetes, lost less in the trials; their numbers are further down this page.

The curve rises fast and then flattens toward the trial’s mean loss. The share of that mean reached by month m is 1 − e^(−k × m), where k = 3 ÷ the trial’s length in months. STEP 1 ran 68 weeks, which the calculator rounds to 16 months; SURMOUNT-1 ran 72 weeks, rounded to 17 months.

Projected loss (%) = trial mean × (1 − e^(−k × m)). Projected weight = starting weight × (1 − projected loss ÷ 100).

Because 1 − e^(−3) = 0.95, the curve reaches 95% of the trial mean at month 16 or 17 and never passes the mean. At the trials' own endpoints it reads 19.9% for tirzepatide and 14.2% for semaglutide, a little under the published means. At month 12 it shows 88% of the SURMOUNT-1 mean and 89.5% of the STEP 1 mean.

Worked example, 220 lb on tirzepatide: k = 3 ÷ 17 = 0.176. At month 12, 1 − e^(−0.176 × 12) = 1 − 0.120 = 0.880. The projected loss is 20.9% × 0.880 = 18.4%, so the weight is 220 × (1 − 0.184) = 179.6 lb. The calculator shows 180 lb and 40 lb lost.

The table runs the same formula from 220 lb at months 3, 6, 9 and 12.

Apart from the 7.2 mg curve, every constant in the calculator comes from one of two trials: SURMOUNT-1 (Jastreboff et al., NEJM 2022) and STEP 1 (Wilding et al., NEJM 2021). The 20-week tirzepatide escalation is from the SURMOUNT-1 paper. The 16-week semaglutide escalation is from the Wegovy prescribing information, section 14, which reports STEP 1 as Study 2.

The trial means already include the escalation weeks, so month 1 of the curve is your first month on the drug, not your first month on the top dose. Both trials enrolled adults without diabetes who had a BMI of 30 or more, or 27 or more with a weight-related condition, and both added lifestyle support.

Besides 15 mg, the calculator offers the SURMOUNT-1 5 mg and 10 mg arms and the 7.2 mg Wegovy dose. SURMOUNT-1 also tested 5 mg and 10 mg of tirzepatide, with mean losses of 15.0% and 19.5% at week 72 (Jastreboff et al., NEJM 2022).

Wegovy now goes up to 7.2 mg for people who have tolerated 2.4 mg for at least 4 weeks. In its 72-week trial, the 7.2 mg group lost 18.8% against 15.5% on 2.4 mg (Wegovy prescribing information, section 2.2 and Table 12). The calculator draws 7.2 mg from that trial, against its 3.9% placebo mean; every brand’s dose steps are in the GLP‑1 dose conversion chart.

The tirzepatide and semaglutide options come from separate trials in different people, so the gap between their curves is not a head-to-head result. In SURMOUNT-5, the direct comparison, tirzepatide gave 20.2% and semaglutide 13.7% at week 72, each at the highest dose people tolerated (Aronne et al., NEJM 2025).

In STEP 4, people on semaglutide 2.4 mg lost 10.6% in a 20-week run-in, then a further 7.9% over the next 48 weeks (Rubino et al., JAMA 2021). In SURMOUNT-4 (Study 4 in the Zepbound label), tirzepatide took off 20.9% in a 36-week lead-in and a further 5.5% from week 36 to week 88 (Zepbound prescribing information, section 14, Study 4).

Both lead-in figures count only the people who went on to randomization: those who reached 2.4 mg in STEP 4, and those on 10 or 15 mg in SURMOUNT-4. The calculator’s curve starts more gently. It puts semaglutide at 8.6% after 20 weeks and tirzepatide at 16.1% after 36 weeks. For the results the trials printed at each time point, see the Wegovy results timeline and Mounjaro weight loss by month.

If your own loss sits below the curve for several weeks, what to check during a weight loss plateau starts with whether the stall is real, then missed doses, the vial, protein and training.

The curve follows the average, and individual results in the trials spread well either side of it. In STEP 1, 86.4% of people on semaglutide lost at least 5%, 69.1% lost at least 10% and 50.5% lost at least 15% (Wilding et al., NEJM 2021). On tirzepatide 15 mg in SURMOUNT-1, 91% lost at least 5% and 57% lost 20% or more (Jastreboff et al., NEJM 2022).

The curve also assumes you keep taking the medicine. In SURMOUNT-4, people switched to placebo after the lead-in gained 14.0% between week 36 and week 88 (Zepbound prescribing information, section 14). The longer trials of stopping are compared in weight regain after stopping.

The GLP‑1 goal weight calculator shows where your 12-month weight sits against a healthy BMI for your height, and the GLP‑1 cost per pound calculator divides your monthly spend by the loss. To compare the projection with your real trend, chart your weigh-ins in Pensday, where weight logging is free.