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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How To Calculate Body Fat Composition
Body fat composition measures the percentage of your total weight that is fat tissue, rather than muscle, bone, and organs. While BMI tells you if your weight is healthy for your height, body fat percentage gives a more detailed picture of your actual body composition. Understanding how to calculate and interpret your body fat is important because two people with the same BMI can have very different health profiles depending on their muscle mass and fat distribution. This guide explains the main calculation methods and what healthy ranges look like for UK adults.
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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.
Methods For Calculating Body Fat Percentage
Several methods exist to measure body fat composition, each with varying accuracy and accessibility.
- DEXA scan (dual-energy X-ray absorptiometry): The most accurate method, measuring bone density and body composition separately. Available at NHS hospitals and private clinics. Considered the gold standard.
- Bioelectrical impedance analysis (BIA): Sends a mild electrical current through your body to estimate fat percentage. Used in gym scales, body composition monitors, and clinical settings. Moderate accuracy but affected by hydration levels.
- Skin fold callipers: A trained technician measures fat folds at specific body sites using callipers, then calculates body fat percentage. Requires skill to perform accurately.
- Waist-to-height ratio: A simple formula — divide your waist circumference by your height (both in the same units). A ratio below 0.5 is considered healthy for all adults and is a reliable predictor of cardiovascular risk.
- Body Mass Index (BMI) combined with waist measurement: While BMI doesn't directly measure body fat, it can be used alongside waist circumference to assess body composition risk more accurately than BMI alone.
Your GP can arrange a DEXA scan if medically indicated. For general tracking, bioelectrical impedance scales offer a practical, affordable option for home use, though results should be interpreted with awareness of their limitations.
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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.
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Healthy Body Fat Ranges And Why BMI Alone Isn't Enough
Healthy body fat percentage ranges (NHS guidance):
- Women: 21–32% (athletic: 14–20%, essential fat: 10–13%)
- Men: 14–24% (athletic: 6–13%, essential fat: 2–5%)
Two people with the same BMI can have very different body compositions. An athlete with high muscle mass may have a BMI in the "overweight" range but very low body fat percentage and excellent health. Conversely, someone with a healthy BMI but low muscle mass and high body fat (sometimes called "normal weight obesity") may have greater health risk. This is why body fat composition matters: it reveals whether your weight is primarily muscle or fat. Combined with waist circumference measurements — over 94cm in men or 80cm in women indicates increased health risk — body fat assessment gives a fuller health picture than BMI alone. Your GP can assess these measures together during a health check.
- 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 whether your total weight is healthy for your height but doesn't distinguish between muscle and fat. Body fat percentage shows exactly how much of your weight is fat tissue. Two people with identical BMI can have very different body fat levels. BMI is useful for screening, but body fat composition gives a more detailed health picture.
Can I calculate my body fat at home?
Yes, using bioelectrical impedance scales (body composition monitors) available from retailers, though accuracy varies. Waist-to-height ratio is a simple calculation you can do yourself — divide waist circumference by height in the same units; below 0.5 is healthy. For the most accurate measurement, ask your GP about DEXA scanning or visit a clinic offering professional body composition analysis.
Is 25% body fat healthy?
For women, 25% falls in the "acceptable" healthy range (21–32%). For men, 25% is slightly above the healthy range (14–24%), approaching the obese category. Health risk depends on age, overall fitness, waist circumference, and other factors. Your GP can advise whether your body fat percentage presents any health concerns in your specific context.
Why does body fat distribution matter?
Where fat is stored affects health risk more than total body fat percentage alone. Visceral fat stored around organs in the abdomen is linked to greater disease risk than fat stored under the skin. Waist circumference is a simple measure of abdominal fat; for UK adults, over 94cm in men or 80cm in women indicates increased cardiovascular risk, regardless of BMI or total body fat percentage.
How often should I measure my body composition?
For home monitoring using scales or waist measurement, every 4–8 weeks is reasonable. More frequent measuring can be demotivating due to normal day-to-day fluctuations in water retention and digestion. If measuring progress during weight loss, track alongside the MyWeightCheck progress tracker to see the bigger picture rather than obsessing over single measurements.
Ready to take control of your weight loss journey?
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