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 Fatness and Why Does It Matter?
Body fatness refers to the proportion of your total body weight that is fat tissue. Unlike BMI, which only uses height and weight, body fat percentage gives a much clearer picture of your actual body composition and health. Understanding your body fatness — alongside your BMI — helps you and your GP assess your real health risk. Even two people with identical BMIs can have very different body fat levels depending on their muscle mass, which is why body composition matters.
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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 Categories
Body fat percentage is calculated as total fat mass divided by total body weight, expressed as a percentage. The healthy ranges vary significantly by age and sex:
- Women aged 20–40: 21–32% body fat is considered acceptable; athletic range is 14–20%
- Men aged 20–40: 8–19% body fat is acceptable; athletic range is 6–13%
- Women aged 41–60: 23–33% body fat is acceptable
- Men aged 41–60: 11–22% body fat is acceptable
Body fat above 32% for women and 25% for men is typically classified as obese. However, these are general guidelines — your GP can advise on what's healthy for your individual circumstances. Use our free BMI calculator alongside an assessment of your body composition for a fuller picture of your health.
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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 Fatness vs BMI and Lean Body Mass
This is where body composition becomes crucial. BMI does not distinguish between muscle and fat — a muscular athlete might have a high BMI but low body fat, whilst someone with low muscle mass could have a normal BMI but high body fatness. Lean body mass (your weight minus fat) includes muscle, bone, and organs. Two people weighing the same can have dramatically different health profiles depending on their ratio of muscle to fat.
Visceral fat — fat stored around internal organs — is particularly concerning for health, even in smaller amounts. This is why body composition assessment, measured through methods like DEXA scans, bioelectrical impedance, or skinfold calipers, is often more informative than BMI alone. Your GP can discuss which assessment method is most appropriate for you.
- 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 is body fat percentage measured?
Several methods exist: DEXA scans (gold standard), bioelectrical impedance analysers (BIA), skinfold calipers, and hydrostatic weighing. BIA devices are available for home use but less accurate than clinical methods. Your GP can refer you for professional assessment if needed for health monitoring.
Can you have high body fatness with a normal BMI?
Yes. This is called "normal weight obesity" — you may have a healthy BMI but high body fat due to low muscle mass. This is why body composition matters alongside BMI. Our free BMI calculator shows your BMI, but body fat assessment requires additional measurement.
What's the difference between visceral fat and subcutaneous fat?
Subcutaneous fat sits under the skin (what you can pinch). Visceral fat wraps around organs and is metabolically active — higher visceral fat increases risk of type 2 diabetes and heart disease even at lower total body fat levels. Waist circumference measurement can indicate visceral fat risk.
How does age affect healthy body fat ranges?
Body fat naturally increases with age. A 60-year-old woman with 35% body fat may be healthier than a 25-year-old with the same percentage. Age-adjusted ranges are more relevant than single standards. Your GP can advise on what's appropriate for your age and health.
Is body fat percentage more important than weight?
Both matter. Weight provides baseline health context, but body composition (body fat vs lean mass) reveals much more about metabolic health and disease risk. The combination of BMI, body fatness, and waist circumference gives the clearest picture — discuss all three with your GP for personalised guidance.
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.