GLP-1 Weight Loss Calculator 2026 — Ozempic & Wegovy

How GLP-1 Medications Work for Weight Loss

GLP-1 receptor agonists are a class of medications originally developed to treat Type 2 diabetes that have since been approved at higher doses specifically for chronic weight management. The name stands for glucagon-like peptide-1, a naturally occurring hormone released by cells in the gut after eating. GLP-1 plays several roles in metabolic regulation: it stimulates insulin secretion from the pancreas, suppresses glucagon release, slows gastric emptying so food moves through the stomach more slowly, and — crucially for weight loss — acts on appetite centres in the brain to reduce hunger and increase feelings of fullness.

Semaglutide, marketed as Ozempic (for diabetes) and Wegovy (for weight management), is a once-weekly injectable GLP-1 receptor agonist. At the 2.4mg dose approved for weight management, it is among the most effective pharmacological treatments for obesity studied to date. Tirzepatide, sold as Mounjaro (for diabetes) and Zepbound (for weight management), takes this further by acting on both the GLP-1 and GIP (glucose-dependent insulinotropic polypeptide) receptors simultaneously — a dual mechanism that clinical trials have shown produces even greater weight loss on average. Both medications also exist in oral formulations though injectable versions remain the standard for weight management in current prescribing practice.

Clinical Trial Results: STEP and SURMOUNT

The weight loss projections in this calculator are grounded in the pivotal randomised controlled trials that led to FDA approval for each medication.

The STEP (Semaglutide Treatment Effect in People with Obesity) trials enrolled thousands of adults with BMI ≥30 (or ≥27 with a weight-related comorbidity). In the landmark STEP 1 trial, participants on semaglutide 2.4mg lost an average of 14.9% of body weight over 68 weeks compared to 2.4% with placebo. The STEP 5 extension confirmed that benefits were sustained at 104 weeks with continued treatment. Across all STEP trials, participants consistently lost between 5% and 17% of their body weight depending on the timepoint and trial design.

The SURMOUNT (A Study of Tirzepatide Once Weekly in Participants With Obesity or Overweight) trials showed even more striking results. In SURMOUNT-1, participants receiving tirzepatide 15mg lost an average of 20.9% of body weight over 72 weeks, with approximately 37% of participants losing 25% or more of their body weight. The 10mg dose produced 19.5% loss and the 5mg dose 15.0%, demonstrating a clear dose-response relationship. These results positioned tirzepatide as the most effective approved pharmacological weight loss treatment to date.

This calculator interpolates between the key clinical data points to give you a week-by-week projection. Because weight loss is not linear — it tends to be faster in the first 12–24 weeks and gradually slows — the underlying data follows the observed trial curves rather than a straight-line assumption.

The Critical Role of Protein During GLP-1 Treatment

One of the most important and underappreciated aspects of GLP-1-assisted weight loss is the need to protect lean muscle mass. When you lose weight rapidly — whether through diet, exercise, or medication — your body does not lose fat exclusively. Without adequate protein intake and resistance exercise, a meaningful proportion of weight loss comes from lean tissue including muscle.

Research suggests that without targeted intervention, lean mass can account for roughly 25–40% of total weight lost during pharmacological weight loss. Given that GLP-1 medications can produce losses of 15–20% of body weight, this represents a clinically significant amount of muscle for many patients. Loss of muscle mass reduces basal metabolic rate (meaning you burn fewer calories at rest), impairs physical function, increases the risk of injury, and may make it harder to maintain weight loss after treatment ends.

Most obesity medicine specialists and registered dietitians now recommend a protein intake of 1.2 to 1.6 grams per kilogram of body weight per day for patients on GLP-1 medications. This calculator provides the lower bound of this range (1.2g/kg) applied to your projected new weight as a minimum daily protein target. Because GLP-1 medications suppress appetite substantially — sometimes making it difficult to eat enough — prioritising protein-dense foods at every meal is essential. Good sources include lean meats, poultry, fish, eggs, dairy, legumes, and protein supplements if dietary intake is insufficient.

Pairing adequate protein intake with two to three sessions of resistance training per week has been shown in multiple studies to significantly reduce the proportion of muscle loss during caloric restriction. This combination represents the current standard of care recommendation in obesity medicine.

Activity Level and Its Effect on Outcomes

Physical activity amplifies the benefits of GLP-1 medications beyond simply burning additional calories. Regular exercise — particularly resistance training — signals the body to preserve muscle tissue even during a caloric deficit. Aerobic exercise improves cardiovascular fitness, insulin sensitivity, and metabolic flexibility. Clinical trial participants with higher physical activity levels tended to achieve greater total weight loss and, more importantly, a more favourable body composition (lower fat percentage, preserved muscle mass) compared to sedentary participants losing the same amount of weight.

This calculator applies a modest activity bonus to the base clinical trial projection: moderate activity (3–5 days per week) adds approximately 1–2% to projected weight loss, and active individuals (6–7 days per week) may see an additional 2–4%. These bonuses are conservative estimates — the actual impact of exercise varies considerably between individuals.

Important Limitations and Considerations

This calculator provides population-level projections based on clinical trial averages. Real-world weight loss with GLP-1 medications varies considerably for several reasons. Dose titration matters — both semaglutide and tirzepatide are titrated slowly over months to their maximum doses to minimise gastrointestinal side effects, meaning the full effect may not be reached until mid-treatment. Side effects including nausea, vomiting, and constipation are common, particularly early on, and can affect how consistently the medication is taken. Insurance coverage and supply availability have historically been inconsistent for these medications in the US. Co-existing medical conditions, concurrent medications, and genetics all influence individual response.

Use this tool as a motivational starting point and to have informed conversations with your healthcare provider — not as a guarantee of outcomes. For a deeper understanding of your weight and health metrics, explore our BMI Calculator and Protein Calculator.

Understanding Your BMI Before and After

BMI (Body Mass Index) is calculated as weight in pounds multiplied by 703, divided by height in inches squared. While BMI has well-known limitations as a sole measure of health, it remains a useful marker for tracking progress and communicating with clinicians. A reduction in BMI of 3–5 points is clinically meaningful and is associated with measurable improvements in blood pressure, blood sugar, cholesterol, and joint load. Moving from one BMI category to a lower one — for example from Obese Class II to Obese Class I, or from Overweight to Normal weight — is a milestone associated with significant reductions in cardiovascular and metabolic disease risk.

To explore related health metrics, check out our Body Roundness Index (BRI) Calculator, which some researchers consider a better predictor of metabolic risk than BMI alone.