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 BMI Index and Percentile Rankings
Your BMI (body mass index) is a key health indicator used by the NHS to assess whether your weight falls within a healthy range for your height. BMI percentile refers to how your BMI compares to the population — but for adults, the NHS uses fixed BMI categories rather than percentiles. Understanding where your BMI sits in relation to NHS thresholds helps you know whether your weight poses health risks and what support might be available. Use our free BMI calculator above to find your personal BMI instantly, then see how it fits into the NHS framework.
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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 They Mean
The NHS uses four main BMI categories for adults, alongside a fifth category for severely obese individuals. These thresholds are based on extensive research into health risks:
- Healthy weight: BMI 18.5–24.9 — associated with lowest health risks
- Overweight: BMI 25.0–29.9 — increased risk of weight-related health conditions
- Obese (Class I): BMI 30.0–34.9 — significant health risks
- Obese (Class II): BMI 35.0–39.9 — higher health risks
- Severely obese (Class III): BMI 40.0+ — substantially increased health risks
For adults of South Asian, Chinese, Black African, and Black Caribbean heritage, the NHS recommends lower thresholds because these groups face higher diabetes and cardiovascular risk at lower BMI levels. For these populations, the overweight threshold starts at BMI 23.0, and obese begins at BMI 27.5. Your GP can explain which thresholds are most relevant for you.
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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.
NHS Weight Management Pathway and When to Seek Help
The NHS offers a four-tier weight management pathway. Tier 1 includes self-help resources and community programmes. If your GP identifies weight-related health risks, they may refer you to Tier 2 (primary care-based support) or Tier 3 (specialist multidisciplinary services where prescription weight loss medications may be considered). Contact your GP if your BMI is 30 or above, or 27–29 with weight-related conditions like type 2 diabetes, high blood pressure, or sleep apnoea. Your GP can assess your individual health and discuss the most appropriate support. Even modest weight loss of 5–10% can significantly improve health markers and reduce disease risk.
- 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's the difference between BMI and BMI percentile?
BMI is a fixed calculation of your weight relative to height. BMI percentile compares your BMI to others — commonly used for children using growth charts. NHS adults use fixed BMI categories (18.5–24.9 for healthy weight, etc.) rather than percentiles. Your personal BMI value tells you which NHS category you fall into.
Is BMI accurate for everyone?
BMI is useful but has limitations. It doesn't distinguish muscle from fat, so athletes may have high BMI but low body fat. It doesn't account for where fat is stored — waist circumference matters too. BMI is less accurate for elderly people, pregnant women, and people with very short or tall statures. Your GP can provide personalised context.
What if my BMI is in the healthy range but I feel unhealthy?
BMI is one health marker among many. Other factors like fitness level, waist circumference, blood pressure, cholesterol, and blood sugar matter significantly. If you have concerns about your health despite a healthy BMI, discuss them with your GP. Use our free BMI calculator and weight converter to track changes over time.
Can I lower my BMI quickly?
The NHS recommends losing weight at a rate of 0.5–1 kg per week for sustainable results. Faster loss often involves losing muscle rather than fat. Sustainable approaches include a modest calorie deficit (around 500 kcal/day), increased physical activity, and behaviour change support. Your GP can discuss appropriate targets for you.
When should I ask my GP about prescription weight loss treatment?
If your BMI is 30 or above, or 27–29 with weight-related health conditions, ask your GP about referral to specialist weight management services. The NHS offers support through structured pathways. Your GP will assess your individual health situation and discuss whether specialist treatment is appropriate for you.
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.