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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Understanding BMI Index Classification and NHS Categories
BMI index classification is a system used by the NHS to categorise adults into weight ranges based on their height and body mass. Your body mass index (BMI) is calculated by dividing your weight in kilograms by your height in metres squared. The NHS uses these classifications to identify whether someone is at a healthy weight or if their weight may pose health risks. Understanding where you fall on the BMI scale is an important first step towards managing your health. Use our free BMI calculator above to find your personal BMI instantly and see which NHS category you fall into.
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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 Classification System
The NHS recognises five main BMI categories for adults aged 18 and over:
- Underweight: BMI below 18.5 — may indicate insufficient weight for your height
- Healthy weight: BMI 18.5 to 24.9 — the optimal range for most adults
- Overweight: BMI 25.0 to 29.9 — increased risk of weight-related health conditions
- Obese (Class I): BMI 30.0 to 34.9 — significant health risks present
- Obese (Class II): BMI 35.0 to 39.9 — higher health risks
- Severely obese (Class III): BMI 40.0 and above — considerable health risks requiring attention
These NHS-endorsed classifications help GPs and healthcare professionals identify whether weight management support may be beneficial. However, the NHS also recognises that BMI has limitations — it does not distinguish between muscle and fat, so athletes or very muscular individuals may have a higher BMI without excess body fat. Your GP can provide personalised guidance about what your BMI classification means for your individual health.
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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 Getting Support
The NHS provides a structured four-tier weight management pathway for adults. Tier 1 includes self-help resources, digital tools like our free BMI calculator and weight converter, and community programmes. Tier 2 offers primary care support through your GP, including behaviour change programmes such as the NHS Weight Loss Plan. Tier 3 is specialist multidisciplinary weight management services, where prescription weight loss medications like Mounjaro or Wegovy may be offered following clinical assessment. Tier 4 is bariatric surgery for severe obesity. Contact your GP if you're concerned about your BMI classification or would like support with weight management — they can assess your individual circumstances and recommend the most appropriate tier of support for you.
- 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 is the BMI formula used by the NHS?
The NHS BMI formula is weight (kg) divided by height (metres) squared. For example, if you weigh 70kg and are 1.73 metres tall, your BMI is 70 ÷ (1.73 × 1.73) = 23.4. Our free BMI calculator above does this automatically — simply enter your height and weight in either metric or imperial units to find your BMI instantly.
Are BMI classifications the same for all ethnic groups in the UK?
No. The NHS recommends lower BMI thresholds for people of South Asian, Chinese, Black African, and Black Caribbean heritage because these groups face higher health risks at lower BMI levels. For these groups, a healthy BMI is 18.5 to 22.9, and overweight starts at 23.0. Always discuss your individual BMI classification with your GP, especially if you belong to one of these groups.
What are the main limitations of BMI classification?
BMI does not account for muscle mass — athletes or very muscular people may have a high BMI despite low body fat. It also doesn't show where fat is stored on your body, and waist circumference is actually a better indicator of health risk. BMI is less accurate for elderly people due to muscle loss and during pregnancy. Use BMI alongside other health measures, not in isolation.
At what BMI should I contact my GP about weight management?
The NHS recommends contacting your GP if your BMI is 30 or above, or if your BMI is 25 to 29.9 and you have weight-related health conditions like diabetes, high blood pressure, or sleep apnoea. Your GP can assess your individual health and discuss whether weight management support would be beneficial for you.
Can I track my progress if I'm working on changing my BMI classification?
Yes. Our free progress tracker allows you to log your weight changes over time and see how your BMI category shifts as you progress. Combining the progress tracker with our BMI calculator helps you monitor your journey and stay motivated as you work towards a healthier weight range.
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.