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
What BMI Is Morbidly Obese?
Morbid obesity, now clinically called "Class III obesity" or "severe obesity" by the NHS, refers to a BMI of 40 or above. This classification indicates a significantly increased risk of serious health conditions including type 2 diabetes, heart disease, and sleep apnoea. Understanding where your BMI falls on the NHS scale is an important first step toward managing your health. Our free BMI calculator can help you check your personal BMI instantly and see where you stand on the official NHS categories.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 Classifications
The NHS uses standardised BMI categories to classify body weight and associated health risks. BMI is calculated using the BMI formula: weight in kilograms divided by height in metres squared.
- Underweight: BMI below 18.5
- Healthy weight: BMI 18.5 to 24.9
- Overweight: BMI 25.0 to 29.9
- Class I Obesity: BMI 30.0 to 34.9
- Class II Obesity: BMI 35.0 to 39.9
- Class III Obesity (Morbid/Severe): BMI 40.0 and above
For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower healthy BMI thresholds because these groups face higher health risks at lower weights. For these populations, the overweight BMI range starts at 23.0 rather than 25.0.
It's important to note that while BMI is a useful screening tool, it has BMI limitations — it does not distinguish between muscle and fat, and does not account for where fat is stored on your body. Your GP can provide personalised health assessment beyond BMI alone.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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
The NHS recognises that severe obesity (Class III, BMI 40+) requires specialist support. The NHS weight management pathway has four tiers: self-help and digital tools (tier 1), primary care interventions (tier 2), specialist multidisciplinary services (tier 3), and bariatric surgery where appropriate (tier 4).
If your BMI is 40 or above, you can speak with your GP about referral to your local tier 3 specialist weight management service. Depending on your circumstances, you may be eligible for prescription weight loss treatment including GLP-1 medications like Mounjaro or Wegovy, alongside lifestyle support. NHS availability varies by region, so early conversation with your GP is important. You can also track your progress with our free progress tracker as you work toward healthier weight.
- 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.
Is there a difference between "morbid obesity" and "severe obesity"?
The NHS now uses the term "Class III obesity" or "severe obesity" rather than "morbid obesity", though the terms refer to the same BMI range (40 and above). The modern terminology is preferred as it removes stigma while maintaining clinical clarity about health risk.
What health risks are associated with a BMI of 40 or above?
A BMI of 40+ significantly increases risk of type 2 diabetes (up to 80 times higher), cardiovascular disease, high blood pressure, sleep apnoea, non-alcoholic fatty liver disease, certain cancers, and mobility problems. Even modest weight loss of 5–10% can meaningfully reduce these risks.
Can I get weight loss medication on the NHS if my BMI is 40?
Yes, you may be eligible. NHS prescription weight loss medications are typically accessed through specialist tier 3 weight management services. Your GP can refer you. Eligibility depends on your individual health circumstances and local NHS availability, which varies significantly by region.
How do I calculate my BMI to see if it's in the Class III range?
Use our free BMI calculator above — simply enter your weight in kg (or stone and pounds) and your height in cm (or feet and inches). The calculator will show your BMI and which NHS category you fall into instantly.
What should I do if my BMI is 40 or above?
Book an appointment with your GP to discuss your health and explore your options. Your GP can assess your overall health, discuss lifestyle changes, arrange referral to specialist services if appropriate, and explain what support is available to you locally through the NHS.
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.