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
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Understanding BMI Index and Morbidly Obese Classification
If your body mass index (BMI) falls into the morbidly obese category, the NHS recognises this as a significant health concern requiring specialist support. A BMI of 40 and above is classified as severely obese (sometimes called "morbidly obese"), and the NHS has clear guidance and support pathways available. Understanding your BMI range and what it means is an important first step. You can use our free BMI calculator above to check your personal BMI instantly, and this guide explains what the NHS recommends at this BMI level.
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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.
NHS BMI Categories and Where Morbidly Obese Fits
The body mass index (BMI) formula is weight (kg) divided by height (m) squared. The NHS recognises specific BMI ranges as health categories:
- Underweight: below 18.5
- Healthy weight: 18.5–24.9
- Overweight: 25.0–29.9
- Obese (Class I): 30.0–34.9
- Obese (Class II): 35.0–39.9
- Severely obese (Class III): 40.0 and above
A BMI of 40 or above is the NHS classification for severely obese, which may also be referred to as "morbidly obese". At this BMI range, health risks from excess weight increase significantly. This includes higher risk of type 2 diabetes, cardiovascular disease, certain cancers, and joint problems. However, the NHS emphasises that support is available, and even modest weight loss can improve health outcomes. NICE guidance recommends specialist intervention at this level.
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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.
NHS Weight Management Pathway and Support Available
The NHS provides a structured weight management pathway with four tiers of support. At a BMI of 40+, you are typically eligible for Tier 3 specialist services — multidisciplinary weight management clinics that may include nutritionists, psychologists, and behaviour change specialists. Your GP can refer you to these services. Some patients may be considered for Tier 4 support, which includes bariatric (weight loss) surgery if appropriate. Access varies by region and NHS waiting lists can be significant, but specialist support is available to help you.
- Contact your GP to discuss referral to specialist weight management services
- Ask about available options in your area, including digital support programmes
- Use our free weight converter to track your weight in kg, stone, and lbs as you progress
- 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 exactly is morbidly obese in terms of BMI?
Morbidly obese refers to a BMI of 40 or above, classified by the NHS as "severely obese (Class III)". This category indicates significant health risk and qualifies for NHS specialist weight management support. At this BMI, even a 5–clinically studied weight loss can bring meaningful health improvements.
How is morbidly obese BMI calculated?
BMI is calculated using the formula: weight (kg) ÷ height (m)². For example, a person who is 170cm tall and weighs 115kg would have a BMI of approximately 39.8, just below the severely obese threshold. Use our free BMI calculator above to find your exact BMI instantly.
What health risks come with a morbidly obese BMI?
At BMI 40+, risks include type 2 diabetes, heart disease, high blood pressure, stroke, sleep apnoea, certain cancers, and joint damage. However, the NHS emphasises that specialist support and weight loss interventions can significantly reduce these risks and improve health outcomes.
What does the NHS recommend if I have a morbidly obese BMI?
The NHS recommends referral to Tier 3 specialist weight management services through your GP. These multidisciplinary teams assess your individual needs and may recommend lifestyle changes, behaviour support, prescription weight loss medication, or bariatric surgery depending on your circumstances.
Can weight loss medication help at a morbidly obese BMI?
Yes. Prescription weight loss medications (GLP-1 injections like Wegovy or tirzepatide like Mounjaro) are available through NHS specialist services for eligible patients, and may support weight loss alongside lifestyle changes. Access requires clinical assessment and varies by region. Your GP referral to specialist services is the first step.
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.