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 Is The BMI For Morbidly Obese?
Morbidly obese is not a standard NHS category, but severe obesity (Class III) begins at a BMI of 40 or above. A BMI of 50 or higher is sometimes referred to as "morbidly obese" in medical literature, though this term is less commonly used in modern UK healthcare. Understanding where your BMI falls within the NHS BMI range UK framework helps you and your GP assess health risks and discuss appropriate support options.
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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 Severe Obesity Thresholds
The NHS BMI formula divides adults into five categories. A BMI of 40 or above is classified as Obese Class III (Severely Obese) — this is the highest standard NHS category. Some medical professionals use the term "morbidly obese" informally for BMI of 50+, but this is not an official NHS BMI category.
- BMI 30.0–34.9: Obese (Class I)
- BMI 35.0–39.9: Obese (Class II)
- BMI 40.0+: Severely Obese (Class III)
- BMI 50+: Sometimes termed "morbidly obese" in clinical settings
At severe obesity, health risks increase significantly — including type 2 diabetes, cardiovascular disease, sleep apnoea, and joint problems. Use our free BMI calculator above to check your personal BMI instantly, and our weight converter to switch between kg, stone, and pounds.
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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.
BMI Limitations and Complementary Health Measures
BMI limitations: BMI does not distinguish between muscle mass and body fat. A very muscular person may have a high BMI but excellent health. BMI also does not account for where fat is stored — waist circumference is equally important. The NHS recommends measuring waist circumference alongside BMI: men over 102cm (40in) and women over 88cm (34.5in) face increased health risks regardless of BMI.
For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower overweight BMI threshold assessments, as these groups face higher diabetes and cardiovascular disease risk at the same BMI. If you have concerns about severe obesity, speak with your GP who can offer personalised health assessment and discuss support options including lifestyle changes, specialist services, or prescription weight management treatment.
- 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.
At what BMI is someone considered morbidly obese?
The NHS does not use "morbidly obese" as an official category. Severe obesity (Class III) begins at BMI 40+. Some clinicians use "morbidly obese" informally for BMI 50+, but this varies. Your GP uses NHS classifications to assess health risk and discuss appropriate support.
What health risks are associated with a BMI of 40 or above?
A BMI of 40+ significantly increases risk of type 2 diabetes, heart disease, high blood pressure, stroke, sleep apnoea, osteoarthritis, and certain cancers. Even a 5–clinically studied weight loss can improve health markers. Your GP can discuss personalised management options.
How do I calculate my BMI to see if I'm severely obese?
BMI = weight (kg) ÷ height (m)². For example, someone weighing 100kg at 1.65m tall: 100 ÷ (1.65)² = 36.7 BMI (Obese Class II). Use our free BMI calculator above for gradual improvement in UK measurements.
What's the difference between BMI categories and actual health risk?
BMI is a screening tool, not a diagnosis. It doesn't measure body fat percentage or where fat is stored. Waist circumference, fitness level, blood pressure, and blood sugar are equally important. Your GP assesses all these factors together for a complete picture.
Are there treatments available for severe obesity in the UK?
Yes. The NHS offers tier 3 specialist weight management services with lifestyle support, dietitian advice, and prescription medications (GLP-1 injections like Wegovy or Mounjaro). Tier 4 services offer bariatric surgery for eligible patients. Access varies by region — speak with your GP about referral options.
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.