GLP-1 Weight Loss Projection Calculator

What the Trials Actually Show

GLP-1 receptor agonists are the biggest shift in obesity medicine in decades, and the trial data is genuinely striking: ~15% average body weight loss for semaglutide (STEP 1, 68 weeks) and ~20% for tirzepatide (SURMOUNT-1, 72 weeks). For comparison, structured lifestyle-only programmes average 3–9% at one year.

Two caveats matter as much as the headline numbers. First, every trial paired medication with a calorie deficit and activity programme — the averages already include lifestyle change. Second, the spread is wide: some participants lost 30%+, while roughly one in seven lost under 5%.

The Part Nobody Markets: Muscle

Rapid weight loss takes muscle with it unless you actively defend it. Analyses of GLP-1 trials suggest 25–40% of weight lost can be lean mass in people who don't strength train or eat enough protein. Losing 15% of your body weight but a third of it as muscle leaves you lighter, weaker, and with a slower metabolism — the classic setup for regain.

• Protein: 1.6–2.2g per kg of bodyweight, every day — harder than it sounds when your appetite is suppressed, which is exactly why it needs planning.
• Resistance training: 2–3 sessions per week is the single strongest signal telling your body to keep muscle.
• Habits for the off-ramp: the STEP extension data shows ~two-thirds regain within a year of stopping. The medication window is when you build the routines that survive it.

Medication or Not, the Physics Don't Change

GLP-1s work primarily by reducing appetite, which makes a calorie deficit dramatically easier to sustain. They don't burn fat directly. That means the playbook is the same with or without a prescription: know your maintenance calories, hold a moderate deficit, hit your protein, lift, and sleep. Use the Calorie Deficit Calculator and Protein Calculator to set those numbers.