Evidence-based weight management
Weight Loss Estimator
See how your current weight compares with published clinical-trial averages at 3, 6, 9 and 12 months. The figures are educational illustrations, not a prediction of your personal result or a treatment recommendation.
Clinical-trial comparison
Illustrative weight-loss averages
Your 5–25% weight-loss milestones
Your target-weight estimate
BMI and eligibility context
For adults, the usual UK marketing-authorisation threshold for these weight-management medicines is an initial BMI of 30 kg/m² or above, or 27 to under 30 kg/m² with at least one weight-related comorbidity. Meeting a BMI threshold does not mean a medicine is suitable or will be prescribed. A prescriber must assess medical history, medicines, contraindications and other clinical factors.
This calculator is designed for adults and does not apply paediatric BMI criteria.
How the estimates are calculated
The model applies the approximate percentage body-weight change shown in major phase 3 trial curves to your starting weight. The 3, 6, 9 and 12-month views are approximate calendar-month checkpoints derived from trial curves, which are reported by study week; the displayed values are curve readings/interpolations rather than guarantees.
The main figure uses the trial-product/efficacy-style curve where available. A lower comparison is derived from the published treatment-policy endpoint for the same study, scaled across the curve. This provides a transparent sensitivity band between two trial analysis approaches; it is not the statistical range of outcomes experienced by individual participants.
Trials differed in design, dose escalation, duration, participant characteristics and lifestyle support. Results from separate trials should not be treated as head-to-head evidence.
Trial sources & important limitations
Mounjaro (tirzepatide) and Wegovy injection (semaglutide 2.4 mg / maximum tolerated dose): SURMOUNT-5, a 72-week head-to-head phase 3b trial in adults with obesity without diabetes. The Wegovy comparator used 1.7 mg or 2.4 mg as the maximum tolerated dose. Wegovy 7.2 mg was not evaluated in SURMOUNT-5 and has since been approved in the UK for some adults with obesity when a higher dose is needed. NEJM 2025. Current UK Mounjaro SmPC. Current UK Wegovy SmPC.
Wegovy tablet (oral semaglutide 25 mg): OASIS 4, a 64-week phase 3 trial. NEJM 2025.
Foundayo (orforglipron): ATTAIN-1, a 72-week phase 3 trial. The displayed trajectory uses the highest-dose efficacy curve as an illustrative benchmark. At week 72, the efficacy estimand was 12.4% mean weight loss; the treatment-regimen estimand was 11.1%. Marketed strength labelling differs from the research-dose labels used in the original trial publication. NEJM 2025.
UK licensing context: MHRA authorisations and current product information should be checked at publication because indications and dose options can change.
Clinical trials report group averages. Real-world results may be lower because of dose tolerance, discontinuation, adherence, health conditions and many other factors. This estimator does not account for diabetes status, previous treatment exposure, dose achieved, adverse effects or treatment interruption.
Next step
Make the treatment decision clinically, not from a calculator
A PrimeLife clinician reviews your health history, current medicines, goals and suitability before discussing any treatment options. Medication is not guaranteed.
Medical disclaimer: This tool is for general information only and does not provide medical advice, diagnose a condition, determine eligibility or recommend a prescription medicine. Always seek advice from an appropriately qualified healthcare professional.
Weight Loss Estimator | Trial-Based Estimates
Use the PrimeLife weight loss estimator to see what published clinical-trial averages could mean at your current weight. Enter your height and weight to calculate your BMI, then explore personalised estimates at 3, 6, 9 and 12 months. You can view results in kilograms or stones and pounds.
The estimator is designed to make percentage weight-loss figures easier to understand. Instead of seeing only a trial average such as 10% or 15%, you can see the equivalent change from your own starting weight. It also shows milestone weights and lets you compare your target with the trial-based estimates.
Important: this is an educational tool. It uses published study averages and does not predict exactly how much weight you will lose. Individual results vary, and treatment suitability can only be decided after a full clinical assessment.
How does the weight loss estimator work?
The weight loss estimator starts with your height and current weight. From these figures it calculates your body mass index (BMI) and converts published average percentage weight changes into an estimated change at your own starting weight.
You can then move between the 3, 6, 9 and 12-month views. The calculator updates the estimated percentage change, the equivalent amount of weight lost and your estimated body weight at that point in time.
This makes clinical-trial results easier to put into context. A percentage can feel abstract. Seeing what the same percentage would mean in kilograms, or in stones and pounds, gives you a clearer reference point.
What does the weight loss estimator show?
After you enter your details, the calculator can show:
- your current BMI and BMI category;
- the weight associated with a BMI of 25 at your height;
- 5%, 10%, 15%, 20% and 25% weight-loss milestones;
- trial-based estimates at 3, 6, 9 and 12 months;
- the equivalent weight change in kg or stones and pounds;
- an optional target-weight comparison; and
- BMI-based information about common adult licence thresholds.
The calculator is intended to help you understand the numbers. It does not select a medicine for you and it does not replace a clinical consultation.
Why use clinical-trial averages?
Clinical trials give us structured information about how groups of people responded to treatment over time. They are useful because they follow defined protocols and measure outcomes consistently.
However, an average is not a promise. Some people lose more than the trial average, some lose less, and some may stop treatment because of side effects, lack of benefit or another clinical reason. Trial populations can also differ in age, starting weight, health conditions and other characteristics.
For that reason, the PrimeLife weight loss estimator uses trial data as an illustrative reference point. It is designed to answer: “What would this average percentage change mean at my starting weight?” It is not designed to answer: “Exactly what will I weigh on a particular date?”
The detailed methodology and study sources used for each treatment are listed inside the estimator.
What might weight loss look like at 3, 6, 9 and 12 months?
3 months
The first few months are often an early-treatment period. Many prescription weight-management treatments use gradual dose increases, so a person may not yet be taking their maintenance dose. Early progress can still be meaningful, but it should not automatically be treated as the final treatment response.
6 months
By six months, there is usually more information about treatment response and tolerability. Weight change may be easier to judge over this longer period than from individual weekly weigh-ins, which can move up and down because of fluid balance, food intake and other short-term factors.
9 months
Nine months gives a longer view of the weight-loss trajectory. The estimator lets you compare the trial-based percentage change at this stage with your own starting weight and target.
12 months
A 12-month view can make longer-term trial averages easier to understand. It is still an estimate rather than a guarantee. Ongoing treatment should be reviewed clinically, with attention to benefits, side effects, nutrition, activity, other medicines and your wider health.
What can affect your actual weight loss?
Two people who start at the same weight can have very different results. Your response can be affected by the treatment and dose you receive, how long you remain on treatment, side effects, missed doses, eating patterns, physical activity, sleep, other medicines and existing health conditions.
Weight also does not usually fall in a perfectly straight line. Short-term changes in hydration, salt intake, bowel contents and hormonal factors can all change the number on the scales. Looking at the overall trend is often more useful than focusing on one weigh-in.
If you are using prescribed weight-management treatment, changes to the dose or treatment plan should be made with your prescriber rather than in response to a calculator result.
What does BMI mean?
BMI is a practical screening measure based on your height and weight. For most adults, NICE classifies a BMI of 18.5 to 24.9 kg/m² as a healthy weight, 25 to 29.9 kg/m² as overweight and 30 kg/m² or above as obesity.
BMI has limitations. It does not directly measure body fat or central adiposity, and it can be less informative in some people, including those with high muscle mass. NICE advises that BMI should be interpreted with caution and, in some adults, alongside measures such as waist-to-height ratio.
Health risks can also occur at lower BMI levels in some ethnic groups. A clinician can interpret BMI in the context of your background, medical history and other risk factors rather than relying on one number alone.
Does the weight loss estimator tell me whether I am eligible for treatment?
No. It provides BMI-based context only.
For several licensed adult weight-management medicines, the usual BMI criteria include a BMI of 30 kg/m² or above, or a BMI from 27 kg/m² to under 30 kg/m² with at least one qualifying weight-related health condition. The exact indication depends on the medicine.
Meeting a BMI criterion does not mean that treatment will automatically be prescribed. A clinician still needs to consider your medical history, current medicines, previous treatment, contraindications, pregnancy or breastfeeding status, side-effect risk and whether treatment can be monitored safely.
For a fuller explanation, read our guide to how medical weight-loss treatments work and who may be eligible.
How accurate is a weight loss estimator?
A weight loss estimator can be useful for translating percentages into real-world numbers, but it cannot know your future response. The figures shown here come from group averages in clinical studies. They are not an individual prediction interval and they should not be used to make treatment decisions without clinical advice.
The most useful way to read your result is as a reference point. It can help you understand what different percentages would mean at your current weight and give you a clearer starting point for a conversation with a clinician.
Frequently asked questions
Is the weight loss estimator medical advice?
No. It is an educational calculator. It does not diagnose a condition, recommend a medicine or replace an assessment by a qualified healthcare professional.
Can I use stones and pounds instead of kilograms?
Yes. Use the unit switch at the top of the calculator to move between metric measurements and UK imperial measurements. The result and chart update to match your selected units.
Why does the calculator show percentages as well as actual weight?
Clinical studies often report weight loss as a percentage of starting body weight. Showing the equivalent amount of weight lost makes that percentage easier to understand for your own starting weight.
Will I definitely reach the estimate shown?
No. The result is based on trial averages. Your own response may be higher, lower or different in timing. Some people may also need to change or stop treatment after clinical review.
Does the calculator choose which treatment is right for me?
No. The comparison is informational. A clinician should decide whether any prescription treatment is suitable after reviewing your health, medicines, goals and treatment history.
Considering medical weight management?
PrimeLife provides a clinician-led online weight-management service for adults. Every patient completes a health assessment and has a mandatory video consultation before any prescribing decision is made. If treatment is appropriate, your progress is reviewed throughout the programme.
Learn more about the PrimeLife Medical Weight Loss Programme
Further reading
- Weight-loss treatments: how they work and who may be eligible
- Weight-loss injection side effects
- How to get weight-loss treatment in the UK
- PrimeLife Medical Weight Loss Programme
Clinical sources
- NICE: identifying and assessing overweight, obesity and central adiposity
- MHRA: GLP-1 medicines for weight loss and diabetes
Medical Disclaimer
This article provides general information and does not replace medical advice. Always speak to your GP, pharmacist or another qualified healthcare professional before you start, stop or change any treatment. A qualified prescriber must assess whether a treatment is suitable for you. PrimeLife only prescribes treatment after a full clinical assessment.
Content created by Jonathan James GPhC 2066318
Last clinically validated on 14/08/2026
