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
What is Normal Body Mass and How to Measure It
Normal body mass refers to a healthy weight range based on your height, age, and body composition rather than weight alone. In the UK, we typically use BMI as a screening tool, but body mass alone doesn't account for muscle versus fat. Understanding your body fat percentage, lean body mass, and where you carry weight gives a much clearer picture of your health. MyWeightCheck provides free tools to help you assess your body composition and see how different metrics work together to give you genuine health insights.
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 Composition Results
The calculator on this page measures body composition, which breaks down your total weight into lean body mass (muscle, bone, organs) and body fat percentage. Healthy body fat ranges vary by age and sex:
- Women aged 20–40: 21–32% body fat is considered healthy (athletic: 14–20%)
- Men aged 20–40: 8–19% body fat is considered healthy (athletic: 6–13%)
- Women over 40: 23–33% body fat is healthy
- Men over 40: 11–22% body fat is healthy
These ranges are based on NHS and clinical guidelines. Your total weight matters, but where your weight comes from — muscle or fat — is equally important for health. A person with high lean body mass and lower body fat may have a higher BMI but be healthier than someone with a lower BMI and higher visceral fat (deep abdominal fat linked to metabolic disease). Use our free BMI calculator above alongside body composition data for the complete picture.
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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 Composition vs BMI — When to Use Each
BMI and body composition measure different things. BMI is quick, free, and useful for population screening — it uses only height and weight. Body composition tells you the quality of your weight and is more accurate for individuals, especially athletes or older adults with muscle loss.
Use BMI when: You want a simple health screening, tracking changes over time, or understanding UK health thresholds.
Use body composition when: You're building muscle, want detailed fat loss tracking, or BMI seems misaligned with how you feel. Try our weight converter above to check where your weight sits across different units, then use body composition for actionable detail. Both tools work best together — neither alone tells your whole story.
- 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 is the difference between body mass and body weight?
Body mass refers to the amount of matter in your body measured in kilograms (or stone and pounds in the UK). Body weight includes everything — muscle, fat, bone, organs, and water. Body composition analysis breaks down that total weight into lean mass (muscle and bone) and body fat percentage, giving you far more useful health information than weight alone.
What is a healthy body fat percentage for UK adults?
Healthy body fat ranges by age and gender. For women aged 20–40, healthy range is 21–32%; for men, 8–19%. These ranges increase slightly with age due to natural muscle loss. The NHS and clinical guidelines recognise these ranges. Very low body fat (<10% for men, <15% for women) is not sustainable and can be harmful.
Is a high BMI always unhealthy if I have lots of muscle?
Not necessarily. BMI doesn't distinguish muscle from fat, so very muscular individuals (athletes, weight lifters) may have a high BMI but low body fat and excellent health. This is where body composition analysis is more useful than BMI alone. Both metrics together give a clearer picture of your actual health status.
How do I measure my body fat percentage at home?
Options include bioelectrical impedance scales, measuring tape methods (waist-to-height ratio), or DEXA scans (most accurate, available privately or through NHS in some cases). Online calculators using measurements can estimate body fat, though accuracy varies. Your GP or a fitness professional can advise on the most suitable method for you.
Why does body composition matter more than just the number on the scales?
Two people weighing the same can have very different body compositions — one might be fit with high lean muscle, the other carrying excess body fat. Body composition reveals your actual health status, metabolic health, and risk of weight-related conditions. It's also crucial for tracking progress during exercise, since muscle weighs more than fat — the scales might not move while your body improves significantly.
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.