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
What Is Body Fat Index and Why Does It Matter?
Body fat index measures the percentage of your total body weight that is fat, offering a clearer picture of your health than BMI alone. While BMI uses height and weight to estimate health risk, body fat percentage directly measures the composition of your body — distinguishing between muscle, bone, and fat. This is particularly important because two people with identical BMI can have very different body fat levels depending on their muscle mass. Understanding your body fat percentage alongside your BMI gives you a more complete assessment of your overall health and fitness.
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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 Fat Percentage Ranges and Healthy Targets
Body fat percentage is categorised into ranges based on age and sex. For women, essential fat (the minimum needed for biological function) is 10–13%, athletic is 14–20%, fitness is 21–24%, acceptable is 25–31%, and obese is 32% or above. For men, essential fat is 2–5%, athletic is 6–13%, fitness is 14–17%, acceptable is 18–24%, and obese is 25% or above.
These ranges reflect natural differences in body composition between sexes. Women require more essential fat for reproductive health, whilst men typically have higher muscle mass. A healthy body fat range for most adults falls within the fitness or acceptable categories — though athletic ranges are achievable with dedicated training.
- Use our free BMI calculator above to check your starting point
- Body fat percentage is more accurate than BMI for athletes and those with high muscle mass
- Measure progress with our free progress tracker to monitor composition changes over time
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Why Body Fat Index Matters More Than BMI Alone
Body fat vs BMI reveals an important distinction: BMI cannot distinguish between muscle and fat. A person with significant muscle mass may have a high BMI but low body fat, whilst someone sedentary might have a normal BMI but elevated body fat — a condition called "normal weight obesity." Visceral fat (fat stored around organs) is particularly concerning for health risk, and body composition analysis helps identify this.
Additionally, lean body mass (muscle, bone, organs) is metabolically active and burns more calories at rest. Higher muscle mass supports better blood sugar control, bone density, and mobility. This is why body fat percentage provides better insight into actual health risk than weight or BMI alone. For a complete picture, use our weight converter to track weight changes in your preferred units alongside body fat monitoring.
- 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.
How do I measure my body fat percentage?
Several methods exist: bioelectrical impedance scales (affordable but less accurate), DEXA scans (gold standard, available at some private clinics), underwater weighing (very accurate but rarely available), and skinfold measurements (requires trained technician). For most people, a quality home impedance scale provides a reasonable estimate for tracking trends over time, though absolute accuracy varies.
What's the difference between body fat index and BMI?
BMI uses only height and weight to estimate health risk, whilst body fat percentage directly measures the proportion of fat in your body. BMI cannot distinguish muscle from fat, making it less accurate for athletes or very muscular individuals. Body fat percentage gives a more detailed picture of body composition and health risk.
Is body fat percentage more important than BMI?
Both are useful together. BMI is a quick screening tool aligned with NHS guidelines, whilst body fat percentage provides specific composition data. Use our BMI calculator to establish baseline health category, then monitor body fat percentage for detailed progress. Neither alone tells the complete story.
What body fat percentage should I aim for?
Healthy ranges depend on age and sex. Generally, fitness (14–20% for women, 6–13% for men) or acceptable (21–31% for women, 14–24% for men) ranges support good health. Your GP can discuss target ranges based on your individual health conditions, fitness goals, and medical history.
Can I have a normal BMI but high body fat?
Yes — this is called "normal weight obesity." Someone sedentary with little muscle mass might have BMI in the healthy range but elevated body fat percentage and associated health risks. This is why tracking body composition alongside weight is important for a complete health picture.
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.