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 Check Your Body Mass and Understand Your Results
Checking your body mass goes beyond just knowing your weight — it's about understanding your body composition. Body mass includes both fat and lean tissue (muscle, bone, organs), and this distinction matters for your health. Unlike BMI, which only uses height and weight, body composition analysis shows how much of your body is actually fat versus muscle. This guide explains how to use a body composition calculator, interpret your results, and understand whether your body mass is in a healthy range 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 Read Your Body Composition Calculator Results
When you use a body composition calculator on this page, you'll receive two key measurements: your body fat percentage and your lean body mass. Body fat percentage is the proportion of your total weight that is fat tissue, whilst lean body mass includes muscle, bone, water, and organs.
- For women: Healthy body fat typically ranges from 21–31%. Essential body fat (the minimum needed for survival) is 10–13%.
- For men: Healthy body fat ranges from 18–24%. Essential body fat is 2–5%.
- Athletic ranges are lower: 14–20% for women, 6–13% for men.
Your body fat percentage becomes more meaningful when tracked over time. A single measurement tells you where you are; repeated measurements show whether your composition is improving through exercise and diet, even if your total weight stays the same.
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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 Mass vs BMI — When Each Matters
Body composition analysis and BMI are complementary tools, not competitors. BMI is a quick screening tool based only on height and weight — it doesn't distinguish between muscle and fat. Someone very muscular might have a "high" BMI but low body fat. Body composition tells the actual story: how much of your body is fat versus muscle.
Use the free BMI calculator above for quick health screening, then use body composition measurement if you want more detail — especially if you're exercising regularly or concerned about visceral fat (deep belly fat that poses metabolic risks). For most people, both metrics together give the clearest picture of health. Your GP can advise which matters most for 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 the difference between body fat percentage and BMI?
BMI uses only height and weight, so it can't tell muscle from fat. Body fat percentage shows the actual composition of your body. A muscular person might have a high BMI but healthy body fat. A sedentary person might have a normal BMI but high body fat. Both are useful — BMI for quick screening, body composition for detail.
What body fat percentage should I aim for?
For women, 21–31% is considered healthy; for men, 18–24%. Athletic individuals often have lower ranges (14–20% for women, 6–13% for men). What matters most is tracking your own trend over time, especially if you're exercising or changing your diet. Your GP can advise on a healthy target for you personally.
Can I have a healthy BMI but unhealthy body composition?
Yes. Someone with a normal BMI but very little exercise might have high body fat and low lean muscle mass. Conversely, a very muscular person might have a high BMI but low body fat. This is why body composition measurement is useful alongside BMI — it shows the full picture.
How often should I check my body composition?
Monthly checks are reasonable if you're actively working on fitness or weight loss. Too-frequent checking (weekly) can be demotivating due to natural fluctuations. Track your progress over weeks and months rather than days. Combined with our free progress tracker, you can spot real trends over time.
Is body fat percentage more important than total weight?
Both matter, but for different reasons. Total weight affects joints and mobility; body fat percentage affects metabolic health. Ideally, both improve together through regular exercise and balanced diet. If only one changes, body composition measurement helps you understand what's actually happening with your fitness progress.
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.