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 Body Composition and Mass Index
Body composition — how much of your weight is fat versus muscle and bone — is one of the most important health indicators, yet it's often overlooked. Whilst BMI gives you a quick screening tool, your actual body fat percentage and lean body mass tell a much more complete story about your health. Whether you're tracking weight loss, building fitness, or managing your health, knowing your body composition helps you understand what's really happening beneath the surface.
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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 Body Composition Results
Body fat percentage is the proportion of your total weight that comes from fat tissue. Reference ranges vary significantly by gender and age:
- Women: Essential fat 10–13%, athletic 14–20%, fitness 21–24%, acceptable 25–31%, obese 32%+
- Men: Essential fat 2–5%, athletic 6–13%, fitness 14–17%, acceptable 18–24%, obese 25%+
Lean body mass includes muscle, bone, organs, and water — everything that isn't fat. A higher lean body mass relative to fat is generally healthier. Remember that muscle weighs more than fat, so two people at the same weight can look completely different depending on their body composition. This is why body fat versus BMI tells such different stories: someone with high muscle mass might have an "overweight" BMI but actually have healthy body composition.
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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.
Body Composition vs BMI — When Each Matters
BMI is useful as a quick health screening tool for most adults, but it doesn't distinguish between muscle and fat. A competitive athlete or weightlifter may have an "overweight" or even "obese" BMI while having very low healthy body fat levels. Conversely, someone with a "healthy" BMI might carry excess visceral fat — the dangerous fat stored around organs — and have poor health markers.
For comprehensive health assessment, use both: check your BMI using our free BMI calculator above, then consider your body composition as part of the bigger picture. Your GP can discuss what's healthy for you personally, especially if you have health conditions or are making significant lifestyle changes.
- 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 a healthy body fat percentage for my age?
Healthy ranges vary by gender and age. For women aged 20–39, aim for 21–32% body fat; men 8–19%. These increase slightly with age. However, "healthy" is individual — an athlete may thrive at lower percentages, while someone with muscle-building goals may have different targets. Your GP can advise what's appropriate for you.
Can I have a healthy BMI but unhealthy body composition?
Yes. Someone with a healthy BMI might have high visceral fat and low lean body mass, whilst an athlete with high muscle may have an "overweight" BMI but excellent body composition. This is why measuring body composition alongside BMI gives a more complete health picture.
How is body fat percentage measured?
Methods include DEXA scans (most accurate), bioelectrical impedance analysers (common in gyms), skinfold calipers, and hydrostatic weighing. Each has different accuracy levels. Your GP or fitness professional can recommend the best method for your needs.
Does body fat percentage matter more than weight on the scale?
Both matter, but differently. Weight alone doesn't tell you if you've lost fat or muscle. During weight loss or fitness training, body composition gives you the real story — you might maintain weight whilst losing fat and gaining muscle, which is far healthier than pure weight loss.
How can I improve my body composition?
Combine resistance training (to build lean body mass) with a moderate calorie deficit and adequate protein intake. Consistency over weeks and months matters more than quick results. Our progress tracker helps you monitor changes — remember that composition shifts happen alongside weight, sometimes before the scale moves.
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.