The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
Free tracker included with every download this week
Biggest Weight Loss — What's Realistic in the UK?
The biggest weight loss people achieve in the UK typically comes from combining lifestyle changes with prescription weight loss medication. Clinical trials show that GLP-1 medications like Wegovy (semaglutide) and Mounjaro (tirzepatide) can help adults lose between 15% and 21% of their body weight when used alongside a reduced-calorie diet and increased physical activity. However, individual results vary significantly, and sustainable weight management requires ongoing commitment beyond medication alone.
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Calculate Your BMI Instantly
Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
How Much Weight Can You Realistically Lose?
The biggest weight loss figures often come from clinical trials, where participants followed structured diet and exercise programmes while taking prescription weight management medication. Wegovy (semaglutide) clinical trials (STEP studies) showed average weight loss of approximately 14.9% of body weight over 68 weeks at the maximum dose. Mounjaro (tirzepatide) trials (SURMOUNT-1) demonstrated average losses ranging from 15% to 21% of body weight, depending on the dose used. For example, a person weighing 100 kg could potentially lose 15–21 kg on average — though actual results vary based on individual factors including diet adherence, physical activity, genetics, and metabolism.
The NHS recommends aiming for weight loss of 0.5–1 kg per week for sustainable results. Very gradual weight management (more than 1.5 kg per week) can lead to muscle loss and nutritional deficiencies, so bigger isn't always better for long-term health. Use our free BMI calculator to understand your current weight status and healthy weight range for your height.
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Achieving Biggest Weight Loss in the UK
To achieve the biggest possible weight loss, the most important factors are consistent calorie deficit and medication adherence. Prescription weight loss medications are available through the NHS via specialist tier 3 weight management services (with variable waiting times across regions) or through private online prescribing. Both require clinical assessment to confirm eligibility — typically a BMI of 30 or above, or 27–29.9 with weight-related health conditions. Our GLP-1 weight loss calculator can help you estimate potential weight loss at different medication doses, though your actual results will depend on your personal circumstances.
Weight loss maintenance is equally important — the biggest challenge isn't reaching maximum weight loss, but keeping it off. This requires ongoing lifestyle changes even after medication ends.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
What's the biggest weight loss possible with Wegovy?
Clinical trials showed Wegovy (semaglutide) helped participants lose an average of 14.9% of their body weight over 68 weeks at the maximum 2.4 mg dose. For a 100 kg person, this could mean approximately 15 kg weight loss. Individual results vary significantly based on diet, exercise, and personal factors.
Can I lose more weight with Mounjaro than Wegovy?
Mounjaro (tirzepatide) showed average weight loss of 15–21% in trials — slightly higher than Wegovy's average 15%. However, comparisons between trials aren't direct, and individual response varies. Your prescriber can discuss which medication might suit you best.
How quickly will I see the biggest weight loss results?
Weight loss typically begins within 2–4 weeks of starting medication, with most significant results occurring between weeks 8 and 16. The NHS recommends losing 0.5–1 kg per week for sustainable, healthy results. Faster loss isn't necessarily better long-term.
Is prescription weight loss medication available on the NHS for biggest weight loss?
Yes, GLP-1 medications are available through NHS specialist weight management services (tier 3). Access varies by region and availability. Your GP can refer you, though waiting times can be lengthy in many areas.
What happens after I stop taking weight loss medication?
Weight loss achieved during medication can be regained if lifestyle changes aren't maintained. The biggest weight loss success stories involve continuing healthy eating and physical activity after medication ends, not relying on the injection alone.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.