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 Figure Out Body Fat Index
Body fat index, or body fat percentage, measures the proportion of fat in your body compared to lean body mass like muscle and bone. Unlike BMI, which only uses height and weight, body fat percentage gives a more detailed picture of your body composition and health risk. This guide explains how to calculate it, what healthy ranges look like in the UK, and why it's worth measuring alongside BMI.
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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.
How to Calculate Body Fat Percentage
There are several methods to measure body fat percentage, ranging from simple to highly accurate. The most common approaches include:
- Skinfold calipers: A GP or trained professional measures skin thickness at multiple sites (usually 3–4 locations) and uses a formula to estimate body fat. This is affordable and widely available on the NHS.
- Bioelectrical impedance analysis (BIA): A device sends a harmless electrical signal through your body to estimate fat mass. Many home scales use this method, though accuracy varies.
- DEXA scan: Uses low-dose X-rays to measure bone density and body composition. Highly accurate but mainly available privately in the UK.
- Hydrostatic weighing: Measures density by weighing you underwater. Very accurate but rarely available outside research settings.
For most people, skinfold calipers or a BIA device provide practical estimates at home or through your GP.
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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.
Healthy Body Fat Ranges and Why BMI Alone Isn't Enough
Healthy body fat ranges vary by sex and age. For women, 21–24% is considered fitness level, whilst 25–31% is acceptable. For men, 14–17% is fitness level and 18–24% is acceptable. Anything above 32% for women or 25% for men may indicate increased health risk.
This is why body fat vs BMI matters: BMI doesn't distinguish between muscle and fat. An athlete might have a high BMI but low body fat, whilst someone with normal BMI could have excess visceral fat (fat around organs) that increases disease risk. Use our free BMI calculator to check your BMI, then measure body fat to get the complete picture of your body composition and health status.
- 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 height and weight only and doesn't account for muscle mass. Body fat percentage shows the actual proportion of fat in your body versus lean mass. Two people with the same BMI can have very different body fat levels. This is why measuring body composition alongside BMI gives you a much clearer picture of your health.
Can I measure body fat at home?
Yes. Bioelectrical impedance scales are affordable and available online, though accuracy varies. Skinfold calipers are also inexpensive but require practice. For the most accurate measurement, ask your GP for a professional skinfold assessment or DEXA scan, which are available through NHS specialist services.
What's a healthy body fat percentage for women and men in the UK?
For women, 21–24% is fitness level and 25–31% is acceptable. For men, 14–17% is fitness level and 18–24% is acceptable. Ranges increase slightly with age. Individual targets should be discussed with your GP based on your health history and goals, not just on generic ranges.
Why does visceral fat matter more than subcutaneous fat?
Visceral fat surrounds your organs and is linked to inflammation, type 2 diabetes, and heart disease. Subcutaneous fat (under your skin) is less metabolically harmful. Body fat tests measure total fat, but waist circumference is a good indicator of visceral fat. Men over 94cm waist and women over 80cm face increased health risk.
Is body fat percentage more important than weight?
Both matter, but for different reasons. Weight tells you the total load on your joints; body composition tells you your metabolic and cardiovascular health risk. Use both measures together. Our free weight converter helps track weight in your preferred units, whilst body fat percentage reveals whether weight changes are muscle or fat loss.
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.