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
Understanding 40 and 10 in UK Health Context
In UK health guidance, the numbers 40 and 10 relate to BMI (Body Mass Index) thresholds and weight classifications. A BMI of 40 or above is classified as severely obese (Class III) according to NHS guidelines, while a BMI of 10 is exceptionally low and would indicate severe underweight status. Understanding these thresholds helps you assess your own health position and determines eligibility for weight management support through the NHS. Using our free BMI calculator above, you can instantly determine your personal BMI and see where you fall within NHS weight categories.
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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 What 40 Means
According to NHS guidelines, a BMI of 40 or above falls into the severely obese (Class III) category. This is the highest BMI classification and indicates significantly elevated health risks including type 2 diabetes, cardiovascular disease, and sleep apnoea.
- BMI 30–34.9 = Obese (Class I)
- BMI 35–39.9 = Obese (Class II)
- BMI 40+ = Severely obese (Class III)
For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds due to higher health risks at equivalent BMI levels. A BMI of 40 in these populations represents an even more significant health concern. Eligibility for prescription weight management medications like Mounjaro or Wegovy typically begins at BMI 30 or above, or BMI 27 with weight-related health conditions such as type 2 diabetes or hypertension.
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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.
UK Weight Management Support at BMI 40
If your BMI is 40 or above, speak to your GP about weight management options available through the NHS. You may be referred to a tier 3 specialist weight management service, which offers personalised support including dietary advice, behavioural coaching, and prescription weight loss medications where appropriate.
The NHS Weight Loss Plan, available free online, provides evidence-based guidance for sustainable weight reduction. Your GP can also assess whether prescription medications are suitable for you based on your individual health profile. Use our weight converter above to track your weight in stone and kilograms, and our free progress tracker to log your journey towards a healthier weight range.
- 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 does a BMI of 40 mean for my health?
A BMI of 40 or above indicates severely obese status and significantly increases risk of type 2 diabetes, heart disease, stroke, and sleep apnoea. You should speak to your GP about weight management support. The good news is that even a 5–10% reduction in body weight can meaningfully improve health outcomes.
Am I eligible for weight loss injections if my BMI is 40?
Yes. Adults with a BMI of 30 or above are typically eligible for prescription weight management medications like Mounjaro or Wegovy through NHS specialist services or private prescribing. Your GP or a licensed prescriber will assess whether this is appropriate for your individual health situation.
What does a BMI of 10 mean?
A BMI of 10 is exceptionally low and indicates severe underweight status. This is rare in adults and typically signals serious malnutrition or underlying medical conditions. Anyone with a BMI this low should seek urgent medical advice from their GP.
How can I calculate my BMI if I weigh 40kg or other values?
Use our free BMI calculator on this page — simply enter your weight in kilograms and your height, and it calculates your BMI instantly. You can also use our weight converter to switch between stone, pounds, and kilograms for reference.
What is the healthy BMI range for UK adults?
According to NHS guidance, a healthy BMI for adults is between 18.5 and 24.9. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the healthy range is 18.5 to 22.9 due to higher health risks at higher BMI levels.
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.