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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NHS Body Mass Calculator — Understanding Your Body Composition
Body mass composition refers to the ratio of fat, muscle, bone, and water in your body—a more detailed picture than BMI alone. The NHS recognises body composition as an important health marker, and our free body mass calculator helps you assess where you sit. Unlike BMI, which only uses height and weight, body composition analysis reveals whether your weight comes from muscle or fat, which matters significantly for your long-term health. This guide explains how to use our calculator and what your results actually mean.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 Mass Calculator Results
Once you've entered your measurements into our free body mass calculator, you'll receive an estimate of your body fat percentage and lean body mass. Body fat percentage is expressed as a percentage of your total body weight—for example, 25% means one quarter of your weight is body fat.
- For women: 21–32% body fat is considered healthy; below 15% is athletic; 33%+ is classified as elevated
- For men: 8–19% body fat is healthy; below 7% is athletic; 25%+ is classified as elevated
These ranges are based on NHS and UK health guidelines. Your lean body mass—muscle, bone, organs, and water—should ideally make up the remainder. Remember that individual variation is significant; age, genetics, and training status all influence what's healthy for you personally. Your GP can provide personalised guidance based on your specific health profile.
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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 uses only height and weight, while body composition analysis reveals what that weight actually comprises. A person with high muscle mass might have a BMI in the "overweight" range but low body fat—a distinction BMI misses. Conversely, someone with a "healthy" BMI might have elevated visceral fat (dangerous fat around organs), invisible to BMI alone.
Use our free BMI calculator alongside this body mass calculator for a complete picture. Body composition is particularly useful if you exercise regularly, are over 65, or have metabolic health concerns. However, BMI remains the standard screening tool in UK healthcare because it's quick and cost-effective. For personalised advice on what your results mean for your health, consult your GP or a registered dietitian.
- 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 fat percentage and BMI?
BMI measures overall weight relative to height; body fat percentage shows how much of that weight is fat versus muscle and bone. Two people with identical BMI can have very different body compositions and health profiles. Body composition is more precise but requires additional measurements.
Why does body fat percentage differ between men and women?
Women naturally carry more essential body fat (for reproductive health) than men—around 10–13% is essential for women versus 2–5% for men. This is why healthy ranges differ. Sex hormones like oestrogen also influence fat distribution and storage patterns.
Can I have a healthy BMI but unhealthy body composition?
Yes. Someone can be metabolically obese (excess body fat) despite a healthy BMI, especially if they're sedentary. Conversely, very muscular individuals might have high BMI but low body fat. This is why using both metrics together—with our free BMI calculator and body mass calculator—gives the clearest picture.
How is body composition measured?
Methods include bioelectrical impedance analysis (BIA, used in our calculator), DEXA scans, underwater weighing, and skinfold measurements. Our calculator uses BIA, which passes a safe electrical current through your body to estimate fat and lean mass. Results are estimates, not exact measurements.
Should I aim for the lowest possible body fat percentage?
No. Essential body fat is vital for organ function, hormone production, and temperature regulation. Going below 15% for women or 7% for men can harm health. Focus on a healthy range for your age and sex, combined with good fitness, rather than an extremely low number.
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.