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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Body Mass and Fat — What's the Difference?
Body mass and body fat are often confused, but they're not the same thing. Body mass is your total weight, including muscle, bone, water, and fat. Body fat percentage measures only the fat tissue in your body. Understanding both gives you a much clearer picture of your health than weight alone. The NHS recognises that body composition matters — two people of identical weight and height can have very different health profiles depending on their muscle and fat distribution. This UK health guide explains what these measurements mean and how to use them alongside BMI for a complete view of your health.
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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.
Body Mass vs Body Fat — The Key Differences
Body mass is your total weight measured in kilograms, stone, or pounds. It includes everything: muscle, bone, water, organs, and fat tissue. When you step on scales, you're measuring body mass — not body fat.
Body fat percentage measures only the fat tissue as a proportion of your total body weight. A person weighing 80kg might have 20% body fat (16kg of fat) or 30% body fat (24kg of fat) — the scales would show the same 80kg, but their body composition is completely different.
- High body mass + low body fat = likely muscular (e.g., athletes, strength trainers)
- High body mass + high body fat = excess fat tissue (increased health risk)
- Normal body mass + high body fat = less visible but still unhealthy fat distribution
This is why BMI alone can be misleading. A muscular person might have a BMI in the "overweight" range but excellent health. Conversely, someone with a "healthy" BMI might carry excess body fat — a condition sometimes called "skinny fat".
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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 Fat Percentage Reference Ranges
The NHS and fitness professionals use these body fat ranges as general guidelines:
- Women: 10–13% essential fat, 14–20% athletic, 21–24% fitness, 25–31% acceptable, 32%+ obese
- Men: 2–5% essential fat, 6–13% athletic, 14–17% fitness, 18–24% acceptable, 25%+ obese
These are reference ranges only — individual targets should be discussed with your GP or a registered health professional. Your waist-to-height ratio (waist circumference ÷ height) is also important: a healthy ratio is below 0.5 for all adults. Use our free BMI calculator and weight converter above to assess your current measurements, then discuss your results with your healthcare provider for personalised guidance on healthy weight management and body composition goals.
- 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 body mass index (BMI) and body fat percentage?
BMI measures weight relative to height and does not distinguish between muscle and fat. Body fat percentage shows exactly how much of your weight is fat tissue. Two people with identical BMI can have very different body composition — one might be muscular, the other might carry excess fat. Both metrics are useful, but together they give a more complete picture than BMI alone.
Can I have a high BMI but low body fat?
Yes. Athletes, bodybuilders, and very muscular individuals often have BMI in the "overweight" or even "obese" range because muscle is denser than fat. This is why your GP may also consider your waist circumference and overall fitness level, not just BMI. Conversely, someone with a "healthy" BMI might have high body fat if they have little muscle mass.
How can I measure my body fat percentage?
Methods include: bioelectrical impedance (scales or handheld devices — convenient but less accurate), skinfold callipers (requires trained professional), DEXA scans (gold standard but not widely available on NHS), and hydrostatic weighing (very accurate but uncommon). For general health purposes, the NHS primarily uses BMI, waist circumference, and your health history. Discuss which method suits your situation with your GP.
Why does body composition matter more than just total body weight?
Your health risk depends on where and how much fat you carry, not just total weight. Excess fat around the abdomen is linked to higher cardiovascular risk than fat on the hips or thighs. Muscle tissue is metabolically active — people with more muscle burn more calories at rest. This is why strength training and weight loss medication (like GLP-1 injections) work best alongside resistance exercise to preserve muscle during weight loss.
How do weight loss medications affect body composition?
GLP-1 weight loss injections like Wegovy help reduce overall body mass by decreasing appetite and food intake. However, gradual weight management can lead to loss of both fat and muscle. The NHS recommends combining medication with resistance training and adequate protein intake to preserve muscle mass during weight loss. Your GP or prescriber can advise on maintaining healthy body composition while losing weight.
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.