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 Does a Morbidly Obese BMI Mean?
A morbidly obese BMI is a health category used by the NHS and healthcare professionals to identify adults at significantly increased risk of serious health conditions. In the UK, a BMI of 40 or above is classified as severely obese (sometimes called "morbidly obese"), indicating that excess weight is likely to have a substantial impact on physical health and wellbeing. Understanding what this means—and what steps are available—is the first step toward positive change.
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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.
BMI Ranges and UK Health Categories
The BMI formula divides your weight in kilograms by your height in metres squared. The NHS uses standardised BMI categories UK to assess health risk:
- 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+ places you in the highest risk category. Use our free BMI calculator above to check your personal BMI instantly—it calculates your exact category and shows where you sit on the healthy BMI range scale. For people of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower overweight BMI threshold and risk thresholds, as these groups face higher health risks at lower BMI levels.
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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 What Else Matters
BMI is a useful screening tool, but BMI limitations are important to understand. BMI does not distinguish between muscle and fat—athletes may have a high BMI but low body fat. It also doesn't account for where fat is stored; waist measurement is equally important for health risk assessment. The NHS BMI guidelines recommend measuring waist circumference alongside BMI: for men, over 102cm (40in) is high risk; for women, over 88cm (34.5in) is high risk.
At a BMI of 40+, your GP may discuss weight management options including lifestyle changes, behaviour support, and in some cases, prescription medication. These conversations should focus on achievable, sustainable changes and managing associated health conditions like type 2 diabetes, high blood pressure, and sleep apnoea.
- 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 health risks come with a BMI of 40 or above?
A severely obese BMI (40+) significantly increases your risk of type 2 diabetes (up to 80 times higher), heart disease, high blood pressure, stroke, certain cancers, osteoarthritis, sleep apnoea, and non-alcoholic fatty liver disease. Even modest weight loss of 5–10% can meaningfully reduce these risks.
How is morbidly obese BMI different from obese BMI?
Obese Class II is a BMI of 35.0–39.9. Severely obese (morbidly obese) is 40.0 and above. The higher category indicates greater health risk and typically qualifies you for NHS specialist weight management services and prescription treatment options.
Can BMI be inaccurate for some people?
Yes. BMI doesn't account for muscle mass, so very muscular people may have a high BMI but low body fat. It's less accurate for elderly people (who lose muscle with age) and doesn't apply to children, who use different age-adjusted charts. Always discuss your personal risk with your GP.
What support is available on the NHS for a BMI of 40+?
The NHS offers specialist tier 3 and tier 4 weight management services, including dietary advice, behaviour support, prescription medications (such as GLP-1 injections), and in some cases, bariatric surgery. Referral is through your GP. Availability varies by region.
Does a high BMI mean weight loss is impossible?
No. Research shows that losing even 5–10% of your body weight can improve blood sugar control, reduce blood pressure, ease joint pain, and lower health risks. Prescription medications, dietary changes, and increased activity—alongside support from your GP—can all help achieve sustainable results.
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.