GLP-1 BMI Calculator

What BMI is and what it isn't

Weight over height squared, in kg/m², and the WHO sorts adults into six bands with it: underweight, normal, overweight, and three classes of obesity [1, 2]. It screens populations; it diagnoses nobody. Muscle and fat weigh the same to it, where the fat sits makes no difference to it, and it misjudges athletes, older adults, and anyone whose build sits outside the middle of the distribution.

None of which stops it mattering. The FDA writes eligibility for these drugs in terms of BMI, so an imperfect measure is the one standing between a patient and a prescription [3, 4].

FDA eligibility for Wegovy and Zepbound

Wegovy at 2.4 mg and Zepbound at 15 mg are approved on identical terms for chronic weight management [3, 4]:

• BMI ≥ 30 (any class of obesity), OR
• BMI ≥ 27 AND at least one weight-related condition alongside it — high blood pressure, sleep apnea, dyslipidemia, type 2 diabetes, or established cardiovascular disease.

Below 27, the labeling does not reach you. Prescribing off-label happens, but rarely for weight management at that end of the range.

What the GLP-1 overlay shows

Underneath the result are cards for each phase 3 trial, showing the BMI and band you would reach on that trial's published mean:

• STEP-1 [5] — 68 weeks on semaglutide at 2.4 mg, for a mean of 14.9%
• SURMOUNT-1 [6] — 72 weeks on tirzepatide at 15 mg, for a mean of 20.9%
• ATTAIN-1 [7] — 72 weeks on orforglipron (Foundayo), for a mean of 12.4%

The arithmetic is plain enough: starting BMI times (1 − mean loss percentage / 100). What any one person actually gets varies a great deal, and our weight loss calculator carries the cautious and optimistic ends of that range, week by week.

Important caveats

• It measures populations, not patients. The same number can sit on two people built entirely differently, carrying entirely different cardiometabolic risk.
• Clearing the label is the start, not the end. Insurers routinely stack their own conditions on top — proof of a lifestyle program, a BMI of 35 rather than 30, particular comorbidities. Our insurance employer checker.
• The spread around a mean is wide. Those figures average thousands of people. Between 10 and 15% of participants lost under 5% even on the maintenance dose, and around 10% lost a great deal more than the mean. Our plateau and non-responder guide for the response distribution.
• Body composition matters more than total weight. As much as 40% of what comes off can be lean tissue where protein and resistance training are neglected. See our muscle mass deep-dive.
• What happens when it stops. Across STEP-4 and SURMOUNT-4, people who came off the drug had put back about two-thirds of what they lost — roughly 67% — inside a year. Our tapering guide.

Important disclaimer

Educational only, and not medical advice. BMI screens, it does not diagnose, and label eligibility is one consideration among many when a prescriber decides whether to start someone on a GLP-1. What individuals actually get varies widely. GLP Watchdog does not diagnose, does not treat, and does not tell you what to take.

Related tools and research

• GLP-1 weight loss calculator — week-by-week trial-data-backed predictions
• GLP-1 dose plotter — how the drug accumulates at each rung of the ladder
• GLP-1 unit converter — milligrams to syringe units, for a compounded vial
• Tirzepatide vs semaglutide head-to-head — full effect-size comparison
• Foundayo (orforglipron) approval deep-dive — the new oral option