What BMI measures and what it misses
Body mass index is weight in kilograms divided by height in meters squared. It is the criterion the FDA labels use for GLP-1 eligibility (30 or higher, or 27 or higher with a weight-related condition) because it is simple and correlates with body fat at the population level. It does not distinguish fat from muscle, does not locate fat, and misclassifies muscular people and some ethnic groups. Clinicians supplement it with waist circumference, blood pressure, glucose and lipids. The calculator reports BMI and the label category, then shows what trial-average weight loss would look like at your weight.
Where the projections come from
Tirzepatide figures are from SURMOUNT-1 (72 weeks; −15.0% at 5 mg, −20.9% at 15 mg; placebo −3.1%). Semaglutide figures are from STEP 1 (68 weeks; −14.9% at 2.4 mg; placebo −2.4%). Both trials enrolled adults without diabetes; adults with type 2 diabetes lose less (SURMOUNT-2: −14.7%; STEP 2: −9.6%). Both included lifestyle counseling. The projection multiplies your weight by the trial mean; the spread around that mean is wide.
What to do with the number
Use it to set expectations and to talk to a clinician, not to choose a dose. The eligibility screener checks contraindications; the timeline tool shows the month-by-month shape of trial weight loss.