How the calculator works
The calculator uses a cubic Hermite spline model fitted to published clinical trial data.
In plain terms, it draws a smooth curve between the weight-loss data points reported at each stage of the trial.
It accounts for the dose titration schedule for each medication. Both Mounjaro and Wegovy start at lower doses and gradually increase, so weight loss accelerates over the first few months before tapering as the body approaches a new set point.
If you have type 2 diabetes, the calculator adjusts its projection downward. People with type 2 diabetes tend to lose less weight on GLP-1 medications, likely because of differences in insulin resistance and starting weight3,4.
The shaded range on the chart represents the approximate 25th to 75th percentile of outcomes, giving you a sense of the typical spread rather than just the average.
The clinical trial data behind the projections
The projections for each medication are based on the landmark registration trials published in the New England Journal of Medicine.
Mounjaro (tirzepatide)
The SURMOUNT-1 trial followed 2,539 adults with obesity over 72 weeks. Participants on the highest dose (15 mg) lost an average of 22.5% of their body weight1.
Mounjaro works as a dual GIP/GLP-1 receptor agonist, meaning it targets two gut hormone pathways rather than one. This is thought to explain its greater efficacy compared to semaglutide alone.
The dose titration schedule starts at 2.5 mg and increases every 4 weeks through 5 mg, 7.5 mg, 10mg, and 12.5 mg before reaching the maintenance dose of 15 mg at around week 20.
For people with type 2 diabetes, the SURMOUNT-2 trial showed average weight loss of around 12.8%, roughly 57% of the weight loss seen in people without diabetes3.
Wegovy (semaglutide)
The STEP UP trial followed 1,407 adults with obesity over 72 weeks. Participants on the highest dose (7.2mg) lost an average of 20.7% of their body weight2.
Wegovy is a GLP-1 receptor agonist that mimics the GLP-1 hormone, reducing appetite and slowing gastric emptying.
The dose titration starts at 0.25mg and increases every four weeks through 0.5mg, 1mg, 1.7mg, and 2.4mg, then continues up to 3.6mg and the maintenance dose of 7.2mg at around week 32.
For people with type 2 diabetes, the STEP-2 trial showed average weight loss of around 9.6% on the 2.4mg dose, roughly 64% of the result in people without diabetes4. Higher doses may improve outcomes in this group, though specific data for the 7.2mg dose in type 2 diabetes is still emerging.
Real-world outcomes from Second Nature
Clinical trials are conducted under controlled conditions with carefully selected participants.
Real-world outcomes can differ because people often change their normal behaviour in a research environment, whereas real-world settings give us a stronger indication of how effective medications are in the more unpredictable environment of people’s day-to-day lives.
In a 12-month retrospective study of 339 Second Nature members published in JMIR Formative Research, participants on tirzepatide lost an average of 22.1% of their body weight, and those on semaglutide lost 17.1%5.
Both figures exceeded the clinical trial benchmarks. Among those on tirzepatide, 95.2% achieved at least 10% weight loss, and 83.7% achieved at least 15%5.
These results reflect the benefit of combining medication with a structured behaviour-change programme.
The participants had access to personalised nutrition guidance, a dedicated health coach, and a curriculum built around habit formation.
The calculator includes these real-world data points alongside the clinical trial projections, so you can see how Second Nature members have performed compared to trial averages.
Factors that affect your results
Your individual weight loss will depend on several factors beyond the medication itself.
Starting BMI
People with higher starting BMIs tend to lose more weight in absolute terms, though percentage weight loss is broadly similar across BMI categories in the clinical trials1,2.
Type 2 diabetes
People with type 2 diabetes consistently lose less weight on GLP-1 medications than those without.
The SURMOUNT-2 and STEP-2 trials both showed this pattern3,4. The calculator applies an adjustment if you indicate you have type 2 diabetes.
Diet and nutrition
Medication reduces hunger so we eat fewer calories, but the quality of our diet is still important to promote all-round health and reduce the risk of muscle loss.
Prioritising protein (1.2 to 1.6g per kilogram of body weight daily) helps preserve muscle mass during weight loss6.
Our free GLP-1 meal plan generator can help with this, creating personalised weekly plans from 687 dietitian-developed recipes.
Physical activity
Regular movement, particularly resistance training, supports muscle maintenance and metabolic health during weight loss.
In the Second Nature study, the proportion of members exercising three or more times per week increased from 35% to 43% over 12 months5.
Adherence and dose
Reaching and maintaining the target dose is important for achieving full results.
Many people experience the most significant weight loss after reaching the maintenance dose, which typically takes 16 to 20 weeks, depending on the medication.
Behaviour change and support
Long-term success appears to depend significantly on whether people build sustainable habits around food, movement, and sleep alongside medication.
The Second Nature study showed that combining medication with a structured programme produced results that matched or exceeded clinical trial outcomes5. This is consistent with broader research showing that behavioural support significantly improves pharmacological weight-loss outcomes.
UK eligibility for GLP-1 medications
In the UK, GLP-1 weight-loss medications are prescribed in line with NICE guidelines. The criteria for both Mounjaro and Wegovy are:
- BMI of 30 or above, or
- BMI of 27 or above with at least one weight-related health condition (such as type 2 diabetes, high blood pressure, or obstructive sleep apnoea)
If your BMI is below 27, you won’t currently be eligible for GLP-1 medication for weight management through the NHS or private prescribers in the UK7,8.
The calculator checks your BMI against these thresholds and will let you know if you fall outside the eligible range.
If you think you may be eligible and want to explore your options, our free health quiz takes two minutes and can help you understand whether Second Nature’s programme is right for you.
Frequently asked questions
How accurate is the weight loss calculator?
The calculator uses data from clinical trials involving thousands of participants and real-world outcomes from Second Nature’s programme.
It provides a reasonable estimate based on averages, but individual results vary significantly depending on adherence, diet, exercise, and biology.
How much weight can you lose on Mounjaro?
In the SURMOUNT-1 clinical trial, participants on the highest dose of Mounjaro (15 mg) lost an average of 22.5% of their body weight over 72 weeks1.
In Second Nature’s real-world programme, members on tirzepatide lost an average of 22.1% over 12 months5.
How much weight can you lose on Wegovy?
In the STEP UP clinical trial, participants on the highest dose of Wegovy (7.2mg) lost an average of 20.7% of their body weight over 72 weeks2.
Second Nature members on semaglutide lost an average of 17.1% over 12 months5.
What BMI do you need for Mounjaro or Wegovy?
Under NICE guidelines, you need a BMI of 30 or above, or a BMI of 27 or above with at least one weight-related health condition7,8.
Is the calculator free?
Yes. The calculator is completely free and doesn’t require an account or sign-up.
Does the calculator account for type 2 diabetes?
Yes. If you indicate you have type 2 diabetes, the projection is adjusted based on the SURMOUNT-2 and STEP-2 trial data, which showed moderately lower average weight loss in this population3,4.
What if I’m already taking GLP-1 medication?
The calculator allows you to select your current dose and how long you’ve been on treatment.
It then adjusts the projection to show your expected results from your current point forward, rather than starting from zero.
Will I regain weight if I stop taking the medication?
Research suggests that a significant proportion of weight lost on GLP-1 medications can be regained after stopping treatment. This is one reason why building sustainable habits around food, movement, and sleep is so important alongside medication.
Can I use this calculator if I’m not in the UK?
The clinical trial data are applicable globally, but the eligibility criteria shown are based on UK NICE guidelines. Prescribing thresholds may differ in other countries.
Take home message
GLP-1 medications can lead to significant weight loss, but results vary depending on the medication, your starting weight, whether you have type 2 diabetes, and the lifestyle changes you make alongside treatment.
This calculator gives you a personalised projection based on the best available evidence from clinical trials and real-world programme data.
If you’re considering GLP-1 medication or already taking it, combining it with structured support can help you get the most from your treatment.
In a 12-month study, Second Nature members on tirzepatide lost an average of 22.1% of their body weight, with 95% achieving at least 10% weight loss5.
Second Nature's Mounjaro and Wegovy programmes
Second Nature provides Mounjaro or Wegovy as part of our Mounjaro and Wegovy weight-loss programmes.
Why choose Second Nature over other medication providers, assuming you're eligible?
Because peace of mind matters.
We've had the privilege of working with the NHS for over eight years, helping people across the UK take meaningful steps toward a healthier, happier life.
Our programmes are designed to meet people where they are, whether that means support with weight loss through compassionate one-to-one health coaching, or access to the latest weight-loss medications (like Mounjaro and Wegovy) delivered alongside expert care from a multidisciplinary team of doctors, psychologists, dietitians, and personal trainers.
At the heart of everything we do is a simple belief: real, lasting change comes from building better habits, not relying on quick fixes. We're here to support that change every step of the way.
With over a decade of experience, thousands of lives changed, and a long-standing record of delivering programmes used by the NHS, we believe we're the UK's most trusted weight-loss programme.
We hope to offer you something invaluable: peace of mind, and the support you need to take that first step.
References
- Jastreboff AM, Aronne LJ, Ahmad NN, et al. (2022). Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine, 387(3), 205-216.
- Wharton S, Freitas P, Hjelmesæth J, et al. (2025). Once-weekly semaglutide 7.2 mg in adults with obesity (STEP UP): a randomised, controlled, phase 3b trial. Lancet Diabetes & Endocrinology, 13(11), 949-963.
- Garvey WT, Frias JP, Jastreboff AM, et al. (2023). Tirzepatide once weekly for the treatment of obesity in people with type 2 diabetes (SURMOUNT-2). Lancet, 402(10402), 613-626.
- Davies M, Færch L, Jeppesen OK, et al. (2021). Semaglutide 2.4 mg once a week in adults with overweight or obesity, and type 2 diabetes (STEP 2). Lancet, 397(10278), 971-984.
- Richards R, Lunt W, Whitman M, Spaltro G, Hall R. (2025). Semaglutide and Tirzepatide in a Remote Weight Management Program: 12-Month Retrospective Observational Study. JMIR Formative Research, 9, e81912.
- Phillips SM, Chevalier S, Leidy HJ. (2016). Protein “requirements” beyond the RDA: implications for optimizing health. Applied Physiology, Nutrition, and Metabolism, 41(5), 565-572.
- National Institute for Health and Care Excellence. (2024). Tirzepatide for managing overweight and obesity. NICE Technology Appraisal Guidance [TA1026].
- National Institute for Health and Care Excellence. (2023). Semaglutide for managing overweight and obesity. NICE Technology Appraisal Guidance [TA875].
