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
Understanding Your Body Fat Index
Your body fat index (or body fat percentage) measures the proportion of fat in your body compared to lean muscle, bone, and organs. Unlike BMI, which uses only height and weight, body fat percentage gives a clearer picture of your actual body composition. The calculator on this page helps you estimate your body fat based on measurements, letting you see where you fall within healthy ranges for your age and gender.
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 Fat Calculator Results
The body fat calculator on this page uses proven estimation methods to give you a personalised result. Here's how to read your score:
- Women: Essential fat is 10–13% (necessary for health), athletic is 14–20%, fitness is 21–24%, acceptable is 25–31%, and above 32% is classified as obese
- Men: Essential fat is 2–5%, athletic is 6–13%, fitness is 14–17%, acceptable is 18–24%, and 25% or above is classified as obese
- These ranges account for age and natural biological differences between genders
- Your individual healthy range depends on your age, genetics, and lifestyle — a prescriptive result from the calculator is a starting point, not a diagnosis
Remember that body fat percentage is more informative than weight alone because it distinguishes between muscle mass and fat mass, giving you a clearer picture of your actual body composition and health status.
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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 Fat Index vs BMI — Which Matters More?
Both metrics are useful but measure different things. BMI (calculated using height and weight) is quick and easy but can't distinguish between muscle and fat — athletes often have high BMI but low body fat. Body fat percentage directly measures what proportion of your body is fat, making it more accurate for understanding true body composition.
The ideal approach is using both together: check your BMI using our free BMI calculator for a quick screening, then track body fat percentage for a more detailed picture. Combined, they give you a complete understanding of whether your weight is healthy for you. Your GP can advise which metric is most relevant to your individual health goals.
- 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 a healthy body fat percentage for my age?
Healthy ranges vary by age and gender. Generally, acceptable body fat for women aged 20–40 is 21–32%, and for men is 8–19%. These ranges increase slightly with age due to natural muscle loss. The calculator on this page provides age-adjusted estimates to help you understand your personal range.
Is body fat percentage more accurate than BMI?
Both metrics serve different purposes. Body fat percentage is better at distinguishing muscle from fat, making it more precise for body composition analysis. BMI is a faster screening tool. Using our free BMI calculator alongside body fat percentage gives you the most complete picture of your health status.
Can I reduce my body fat percentage without losing weight?
Yes. Building muscle through strength training while maintaining the same weight can lower your body fat percentage because muscle is denser than fat. This is why the number on the scales might not change, but your body composition improves significantly.
How often should I measure my body fat?
Monthly measurements are reasonable for tracking progress. However, body fat percentage naturally fluctuates day-to-day due to water retention and hormones, so avoid obsessive daily checking. Focus on trends over weeks and months rather than individual results.
What is visceral fat and why does it matter?
Visceral fat is fat stored around your organs inside the abdomen. It's linked to higher health risks than subcutaneous fat (fat under the skin), even at lower total body fat percentages. High visceral fat increases risk of type 2 diabetes and heart disease, which is why waist circumference is also important alongside body fat percentage.
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.