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 a BMI Over 40 — NHS Health Guidance
A BMI over 40 is classified as severely obese according to NHS guidelines and indicates a significantly increased risk of serious health conditions. If your BMI falls above 40, the NHS recommends speaking with your GP about weight management support and health screening. This guide explains what a BMI over 40 means, the NHS classification system, and the healthcare pathways available to you in the UK. You can check your personal BMI instantly using our free BMI calculator above.
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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.
NHS BMI Categories and Classification
The NHS BMI formula divides adults into five categories based on their weight and height. A BMI over 40 falls into the severely obese (Class III) category, which carries the highest health risk among adult weight classifications.
- BMI 18.5–24.9: Healthy weight (NHS recommended range)
- BMI 25.0–29.9: Overweight
- BMI 30.0–34.9: Obese (Class I)
- BMI 35.0–39.9: Obese (Class II)
- BMI 40.0+: Severely obese (Class III) — highest risk category
For people of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower BMI thresholds due to higher cardiovascular and diabetes risk at the same BMI. Your GP can discuss whether ethnicity-adjusted thresholds apply to you. Use our free weight converter to check your weight in different units (kg, stone, and lbs).
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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 Support and Next Steps
If your BMI is over 40, the NHS recommends contacting your GP as a first step. Your GP can assess your overall health, check for weight-related conditions like type 2 diabetes and high blood pressure, and discuss whether you're eligible for specialist weight management support.
The NHS offers a four-tier weight management pathway: tier 1 includes digital tools and community programmes; tier 2 provides GP-led behaviour change support; tier 3 offers specialist multidisciplinary services (where prescription weight loss medications may be considered); and tier 4 covers bariatric surgery for severe obesity with significant health complications. Availability varies by region, and waiting times can be substantial. Your GP can refer you to the appropriate tier based on your individual circumstances and local services.
- 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 are the health risks of having a BMI over 40?
A BMI over 40 significantly increases your risk of type 2 diabetes, cardiovascular disease, certain cancers, sleep apnoea, high blood pressure, and osteoarthritis. Even modest weight loss of 5–10% can meaningfully improve these risk factors. Your GP can discuss screening for weight-related conditions.
Does BMI over 40 mean I definitely have health problems?
Not necessarily — some people with high BMI remain relatively healthy, whilst others experience complications. BMI is a screening tool, not a diagnosis. Your GP will assess your individual health, blood pressure, cholesterol, blood sugar, and other factors to understand your personal risk.
Can I access weight loss medication through the NHS if my BMI is over 40?
Potentially yes. Adults with a BMI of 30 or above may be eligible for prescription weight management medication through NHS tier 3 specialist services, but eligibility depends on clinical assessment, weight-related comorbidities, and local availability. Your GP can refer you for assessment. Waiting times vary significantly by region.
How can I lower my BMI from over 40?
Sustainable approaches include a reduced-calorie diet (typically 500 kcal deficit daily for ~0.5 kg/week loss), increased physical activity, and behaviour change support. The NHS recommends losing no more than 0.5–1 kg per week. Your GP can refer you to tier 1 or 2 NHS programmes. Prescription medication is available for some individuals through specialist services.
What is the difference between BMI over 40 and other obesity categories?
BMI 30–34.9 is Class I obesity, BMI 35–39.9 is Class II obesity, and BMI 40+ is Class III (severely obese). Class III carries the highest risk of serious health complications. The NHS offers increasingly intensive support as BMI increases. Your GP can discuss which support pathway is most appropriate for you.
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.