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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How to Compute Body Mass Index — Your Complete Guide
Body mass index (BMI) is a simple calculation that uses your height and weight to estimate whether you're in a healthy weight range. Computing your BMI takes just seconds using the calculator on this page. However, BMI is just one part of assessing your health — body composition, including your body fat percentage and lean body mass, tells a more complete story. The NHS uses BMI as a screening tool alongside other measurements like waist circumference. Let's explore how to use the calculator correctly and what your results actually mean for your 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.
How to Interpret Your BMI Calculator Results
Once you've computed your BMI using the calculator on this page, you'll see a result between roughly 15 and 40. Here's what the NHS categories mean:
- Below 18.5: Underweight — consider speaking with your GP about nutrition
- 18.5–24.9: Healthy weight — this is the target range for most UK adults
- 25.0–29.9: Overweight — increased health risks begin to rise
- 30.0–34.9: Obese (Class I) — healthcare intervention often recommended
- 35.0–39.9: Obese (Class II) — higher cardiovascular and metabolic risk
- 40.0+: Severely obese (Class III) — significant health risks present
If you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds: healthy weight is 18.5–22.9, and overweight starts at 23. This is because these populations face higher risks of type 2 diabetes and cardiovascular disease at lower BMI levels. Use our free BMI calculator above to check where you fall, then discuss results with your GP if you have concerns.
Clinically BackedWeight Loss
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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 vs Body Composition — When Each Matters Most
BMI is useful but incomplete. Your BMI computation doesn't distinguish between muscle and fat — a muscular athlete might have a "high" BMI despite low body fat percentage. This is where body composition analysis becomes valuable. Healthy body fat ranges vary by sex: women typically have 21–32% body fat in the "acceptable" range, whilst men's acceptable range is 18–24%. Visceral fat (deep abdominal fat) is more concerning for health than subcutaneous fat, but BMI can't measure it. Your GP or a weight management service can assess body composition through bioelectrical impedance or DEXA scans if needed. Use the weight converter and BMI calculator together to understand your full picture — compute your BMI, then consider waist circumference (over 94cm in men or 80cm in women signals increased risk) for a more complete 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's the formula for computing body mass index?
BMI = weight (kg) ÷ height (m)². For example, if you weigh 70kg and are 1.75m tall, your BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = approximately 22.9. Our free BMI calculator does this instantly — just enter your details above.
Does a higher BMI always mean I'm unhealthy?
Not necessarily. BMI doesn't account for muscle mass, which is denser than fat. Athletes often have high BMI but low body fat percentage. However, for most people, BMI correlates with health risk. Your GP can assess your individual health context alongside other factors like body fat vs BMI measurements and waist circumference.
Why are BMI thresholds different for different ethnic groups?
Research shows South Asian, Chinese, Black African, and Black Caribbean adults have higher risk of type 2 diabetes and heart disease at lower BMI levels than European populations. The NHS therefore recommends lower thresholds for these groups: healthy weight starts at BMI 18.5–22.9 rather than 18.5–24.9.
Can I compute BMI if I use stone and pounds instead of kilograms?
Yes. Use our weight converter to convert stone and pounds to kilograms first, then calculate BMI. Or use our free BMI calculator above — it accepts both metric and imperial measurements and computes your result instantly in either system.
How often should I compute my BMI?
Computing BMI monthly is reasonable if you're actively managing your weight. However, weight fluctuates daily due to water retention and food intake, so monthly or quarterly checks are more meaningful than daily tracking. Use our free progress tracker to log changes over time and spot trends.
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.