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
Work Out Your Body Mass and Body Composition
Understanding your body mass goes beyond weight alone. Body composition measures the ratio of fat to lean body mass — muscle, bone, water, and organs — giving you a clearer picture of your health than weight or BMI alone. This guide explains how to interpret body composition results, what healthy body fat percentage ranges look like for different genders, and why this metric matters alongside BMI for UK adults.
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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 composition is typically expressed as body fat percentage — the percentage of your total weight that comes from fat tissue. Use our body fat calculator above to determine your estimated body fat percentage based on your measurements.
- For men: Essential fat is 2–5%, athletic range is 6–13%, fitness is 14–17%, acceptable is 18–24%, and obese is 25% or above
- For women: Essential fat is 10–13%, athletic is 14–20%, fitness is 21–24%, acceptable is 25–31%, and obese is 32% or above
These ranges account for the fact that women naturally carry more essential fat for reproductive health. Individual results vary based on age, genetics, and training status. Your GP can provide personalised advice about what healthy body fat means for 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
Body composition and BMI serve different purposes. BMI (body mass index) is a quick screening tool based only on height and weight — it doesn't distinguish between muscle and fat. This means an athlete with significant lean body mass might have a "high" BMI but very low body fat percentage.
Body composition provides a more detailed picture by measuring actual fat distribution, including visceral fat (fat around organs) which is linked to metabolic disease. Use our free BMI calculator alongside body composition assessment for complete health context. For most UK adults, the combination of both metrics gives the clearest view of health risk.
- 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 and body weight?
Body weight is your total weight in kg or stone. Body mass composition breaks that weight into fat mass and lean body mass (muscle, bone, water, organs). You can weigh the same as someone else but have very different body compositions. Body composition is more relevant to health risk than weight alone.
Can I have a healthy BMI but poor body composition?
Yes. Someone with a healthy BMI might have high body fat percentage if they have little muscle mass. Conversely, muscular individuals may have elevated BMI but low body fat. This is why body composition and BMI together give a more complete picture of health for UK adults.
Why do men and women have different healthy body fat ranges?
Women naturally carry more essential fat for reproductive and hormonal health — around 10–13% compared to men's 2–5%. This isn't a health difference; it's biological. Both genders have similar acceptable ranges (18–24% for men, 25–31% for women) when accounting for this difference.
How often should I measure body composition?
Measure every 4 weeks if tracking progress, or every 3 months for general health monitoring. Frequent measurements (weekly) can be misleading due to water retention and daily fluctuations. Keep a record and look for trends over months rather than focusing on single measurements.
Is body composition more important than BMI?
Neither is more important — they measure different things. BMI is a useful screening tool for health risk at population level. Body composition provides individual detail about where your weight comes from. Use both together alongside waist circumference for the most complete picture of your health.
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.