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
BMI Stands For: Body Mass Index Explained
BMI stands for Body Mass Index — a simple measurement that uses your height and weight to estimate whether you're at a healthy weight. The NHS uses BMI as a screening tool to assess health risk in UK adults, though it's important to understand what it does and doesn't measure. BMI is calculated using a standard formula and falls into defined categories that guide health recommendations. While useful, BMI has real limitations that your GP can help you understand in the context of your overall health.
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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 divides BMI into clear categories to help assess health risk. A healthy BMI ranges from 18.5 to 24.9, which is the target range the NHS recommends for most UK adults. Overweight is classified as BMI 25.0 to 29.9, while obese starts at 30.0. Above 40.0 is classed as severely obese. However, BMI thresholds are lower for some groups: the NHS recommends adults of South Asian, Chinese, Black African, or Black Caribbean heritage use thresholds starting at 23.0 for overweight and 27.5 for obese, because these populations face higher health risks at lower BMI levels. NICE guidance emphasises that while BMI is a useful screening tool, it should never be used alone — your GP will consider your overall health picture, including where fat is stored (waist measurement), fitness level, and any existing health conditions.
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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 Support and When to Seek Help
If your BMI falls outside the healthy range, the NHS offers support through its four-tier weight management pathway. Tier 1 covers self-help tools and community programmes — MyWeightCheck's free BMI calculator can help you track where you stand. Tier 2 involves primary care interventions through your GP, including behaviour change support. Tier 3 offers specialist multidisciplinary weight management services where prescription weight loss treatment may be considered if appropriate. Tier 4 covers bariatric surgery for severe obesity with significant health complications. Your GP is the right person to discuss your BMI result and what it means for your individual health — they can also refer you to appropriate support if needed.
- 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 BMI actually measure?
BMI uses your weight and height to estimate body fatness using a simple formula: weight (kg) divided by height in metres squared. It's a screening tool, not a diagnosis. It doesn't distinguish between muscle and fat, or measure where fat is stored, which is why your GP considers other factors too.
Is BMI the same for everyone in the UK?
No. The NHS recommends lower BMI thresholds for people of South Asian, Chinese, Black African, or Black Caribbean heritage because these groups face higher cardiovascular and diabetes risk at the same BMI as white European adults. Always discuss your personal thresholds with your GP.
What are the limitations of BMI?
BMI doesn't account for muscle mass (athletes may have high BMI but low body fat), bone density, age, or where fat is stored. It's less accurate in elderly people, children, and pregnant women. It's a useful screening tool but never the complete picture of health.
How do I calculate my BMI using the NHS method?
The NHS BMI formula is: weight in kg ÷ (height in metres)². Use our free BMI calculator on this page to calculate instantly. Alternatively, divide your weight in kg by your height in metres, then divide again by your height in metres.
Should I be concerned if my BMI is outside the healthy range?
A BMI outside 18.5–24.9 may indicate health risk, but it's not a diagnosis. Schedule a GP appointment to discuss your result in context — they'll consider your overall health, fitness, family history, and other factors before recommending any changes or support.
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.