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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BMI for Morbidly Obese: What the Numbers Mean
Morbidly obese refers to severe obesity with significant health risks. In the UK, a BMI of 40 or above falls into the severely obese category — though some healthcare professionals use "morbidly obese" to describe a BMI above 35, particularly when combined with serious weight-related health conditions. Understanding your BMI range is crucial for making informed health decisions. Our free BMI calculator above can help you find your personal BMI instantly, and we've explained what those numbers mean for your health below.
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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 Morbid Obesity Thresholds
The NHS BMI range system classifies adults into five main categories based on health risk:
- Underweight: Below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25.0 to 29.9
- Obese (Class I): 30.0 to 34.9
- Obese (Class II): 35.0 to 39.9
- Severely obese (Class III): 40.0 and above
A BMI of 40 or above is classified as severely obese — the highest risk category recognised by the NHS. This threshold indicates significantly elevated risk of type 2 diabetes, cardiovascular disease, and other serious health conditions. Some clinicians refer to this as "morbidly obese" because the health risks become acute. The bmi formula (weight in kg divided by height in metres squared) applies equally — at a severely obese BMI, urgent medical support is typically recommended. Your GP can assess your specific situation and discuss appropriate next steps.
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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 are important to understand: BMI does not distinguish between muscle and fat, and does not account for where fat is stored (visceral fat around organs is more dangerous than subcutaneous fat). At severe obesity levels, BMI is still a useful screening tool, but should always be combined with other measurements. Your GP may also assess your waist circumference — a waist circumference above 102cm (40 inches) in men or 88cm (34.5 inches) in women indicates significant health risk even if BMI alone seems borderline. Blood pressure, blood sugar levels, and cholesterol are also critical indicators. Use our free weight converter to track changes in kg or stone, and consider speaking with your GP about a formal health assessment.
- 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 BMI number counts as morbidly obese?
A BMI of 40 or above is classified as severely obese by the NHS. Some healthcare professionals use "morbidly obese" to describe BMI above 35, particularly if serious weight-related health conditions are present. The exact threshold can vary slightly between clinicians, but BMI 40+ always requires medical support.
What are the health risks at a severely obese BMI?
Severely obese BMI dramatically increases risk of type 2 diabetes, heart disease, stroke, sleep apnoea, non-alcoholic fatty liver disease, certain cancers, and joint problems. At this BMI level, quality of life and life expectancy can be significantly affected. Your GP can assess your individual health risks.
Can I calculate my own BMI for morbid obesity?
Yes. Use our free BMI calculator above, or calculate manually: divide your weight in kg by your height in metres squared. For example, someone weighing 120kg and 1.75m tall has a BMI of about 39.2. If your result is 40 or above, speak with your GP about health support options.
Is BMI the only measure doctors use for severe obesity?
No. BMI is a screening tool, but doctors also assess waist circumference, blood pressure, blood sugar, cholesterol, and other health markers. Someone at a severely obese BMI may have different health risks depending on fitness level, medical history, and where fat is stored.
What treatment options exist for severe obesity?
Options include lifestyle changes (diet and exercise), prescription weight loss medications (available through NHS specialist services or private prescribers), and in some cases, bariatric surgery. Your GP can refer you to specialist weight management services. Individual suitability depends on your health profile.
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.