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
BMI vs Body Fat — NHS Context and When Each Matters
BMI and body fat percentage are different measurements that tell you different things about your health. While the NHS uses BMI as a primary screening tool, body fat percentage gives additional context about your body composition. Understanding both — and their limitations — helps you get a clearer picture of your weight and health status. Let's look at how each measurement works and why the NHS recommends considering both.
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 BMI and Body Fat Differ — NHS Approach
Body Mass Index (BMI) is a simple calculation based on your height and weight: weight (kg) ÷ height (m)². The NHS uses BMI categories to screen for health risks, with a healthy BMI range of 18.5 to 24.9 for most UK adults. It's quick, free, and useful for population-level screening.
Body fat percentage measures the proportion of fat in your body relative to muscle, bone, and water. It requires more specialist measurement (DEXA scan, bioelectrical impedance, or skinfold testing) and gives a more detailed picture of your body composition.
- BMI does not distinguish between muscle and fat — an athlete with high muscle mass may have a high BMI but low body fat
- Body fat percentage reveals muscle quality but requires specialist assessment
- NHS primarily uses BMI for screening because it is accessible and evidence-based for population risk assessment
- The body mass index formula is the same for all adults, but body fat assessment varies by age, sex, and measurement method
Use our free BMI calculator above to find your personal BMI quickly, then consider discussing body composition with your GP if you want a fuller picture of your health.
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.
NHS Weight Management — Using Both Measurements
The NHS weight management pathway starts with BMI screening at your GP. If your BMI falls into the overweight (25–29.9) or obese (30+) category, your GP may discuss your health risks and offer support through tier 1 and tier 2 programmes — including diet and lifestyle advice.
For most people, improving your health means reaching a healthy BMI range: 18.5 to 24.9. However, the NHS also recognises that waist circumference matters — excess fat around your middle carries higher health risk than fat elsewhere. If you're concerned about your body composition or how your weight affects your health, your GP can refer you to specialist services for more detailed assessment, which may include body fat testing.
Track your weight loss progress using our free progress tracker as part of your health journey.
- 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.
Why does the NHS use BMI instead of body fat percentage?
BMI is quick, non-invasive, and evidence-based for population health screening. Body fat measurement requires specialist equipment and training, making it less practical for routine GP assessment. The NHS uses BMI as an initial screening tool; body fat can be assessed by specialists if needed for detailed health planning.
Can I have a high BMI but low body fat?
Yes, if you have high muscle mass — athletes and people who strength train regularly may have BMI in the overweight range but low body fat. This is why the NHS recommends discussing your individual health with your GP rather than relying on BMI alone, especially if you're very muscular.
What's the healthy body fat percentage range?
For women, 21–24% is considered "fitness" level and 25–31% is "acceptable". For men, 14–17% is "fitness" and 18–24% is "acceptable". However, healthy body fat depends on age, genetics, and individual health. Your GP or a qualified health professional can advise what's right for you.
When should I ask my GP about body fat testing?
If your BMI is in the healthy range but you have concerns about your body composition, or if you want detailed assessment for a weight loss or fitness programme, ask your GP. They can refer you to a specialist if needed, though this may not be routinely available on the NHS depending on your region.
Does reducing weight always mean reducing body fat?
Not necessarily — gradual weight management can include muscle loss as well as fat loss. The NHS recommends losing weight gradually (0.5–1 kg per week) alongside physical activity to preserve muscle. A healthy approach combines calorie reduction with strength and aerobic exercise for better body composition outcomes.
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.