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 Body Composition: Beyond Simple Weight Measurements
A "body generator" isn't a medical term, but it refers to how your body builds and maintains muscle, fat, and bone tissue—your body composition. While BMI gives you a single number based on height and weight, body composition tells you what that weight actually consists of. This distinction matters because two people at the same weight can look and feel completely different depending on their muscle-to-fat ratio. Our UK health guide explains why body composition matters for your wellbeing and how to assess it alongside standard weight measurements.
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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.
Why Body Composition Matters More Than Weight Alone
Your body composition is the percentage of your body made up of fat, muscle, bone, and water. Unlike BMI, which treats all weight equally, body composition reveals whether you're carrying excess fat or healthy muscle mass. This matters because muscle is metabolically active—it burns more calories at rest than fat tissue does.
For example, an athlete and someone with excess weight might have identical BMI readings, but very different health profiles. The athlete may carry more muscle (which is denser than fat), whilst the other person carries more body fat. According to NHS guidance, healthy body fat ranges for UK adults are:
- Women: 21–24% (fitness level); 25–31% (acceptable)
- Men: 14–17% (fitness level); 18–24% (acceptable)
You can estimate your body composition using our free BMI calculator, which provides context about healthy weight ranges for your height, though direct body fat measurement requires specialist equipment like DEXA scans or bioelectrical impedance analysis.
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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.
Practical Ways to Build and Maintain Healthy Body Composition
Building lean muscle and reducing excess body fat involves both diet and physical activity. The key is consuming adequate protein (1.2–2g per kg body weight if strength training) whilst maintaining a modest calorie deficit. Resistance training—whether weights, bodyweight exercises, or resistance bands—preserves muscle during weight loss.
If you're managing your weight through lifestyle changes or considering weight loss medication, tracking body composition alongside weight gives you a clearer picture of progress. Use our free progress tracker to log your weight weekly, and pair this with measurements of your waist circumference—research shows waist-to-height ratio is a strong indicator of cardiovascular health, independent of BMI. For personalised guidance on healthy weight ranges for your specific height and background, try our free BMI calculator above.
- 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 BMI and body composition?
BMI is a simple calculation based on height and weight alone—it doesn't distinguish muscle from fat. Body composition reveals what your weight actually consists of. Two people with identical BMI can have very different muscle-to-fat ratios. Body composition is more useful for understanding your actual health profile, but BMI remains a helpful initial screening tool.
How can I measure my body composition at home?
Simple home methods include waist circumference measurement, waist-to-height ratio, and bioelectrical impedance scales (though accuracy varies). The most accurate methods require professional assessment: DEXA scans (gold standard), hydrostatic weighing, or Bod Pod testing. Your GP can refer you for professional measurement if clinically relevant.
Does losing weight mean I'm losing body fat?
Not necessarily. gradual weight management often includes muscle loss alongside fat loss. This is why strength training during weight loss is important—it helps preserve muscle whilst reducing body fat. Gradual weight loss (0.5–1kg per week) minimises muscle loss. Our progress tracker helps you monitor your journey beyond just the scales.
Can someone be overweight by BMI but healthy by body composition?
Yes. An athletic person with significant muscle mass might have a BMI in the "overweight" range but very low body fat percentage. However, excess body fat (regardless of BMI) increases health risks. For accurate assessment, combine BMI with waist circumference and, if possible, body fat measurement.
How does building muscle affect weight loss efforts?
Muscle is denser than fat, so building muscle whilst losing fat can mean your weight changes slowly even though your body composition improves significantly. This is why scales alone don't tell the whole story. Combining strength training with a modest calorie deficit achieves better body composition changes than diet alone.
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.