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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BMI and Body Fat Calculator — NHS Health Guide
Understanding both your BMI and body fat percentage gives you a complete picture of your health. While BMI (body mass index) is a quick screening tool used by the NHS, it doesn't tell the whole story — body fat percentage reveals how much of your weight is actually fat versus muscle. This guide explains NHS BMI categories, body composition basics, and how to use both measurements together for better health decisions. Use our free BMI calculator above to check your personal BMI instantly, then explore what it means for your health using NHS guidelines.
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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.
NHS BMI Categories and What They Mean
The NHS uses five BMI categories to classify adult weight status. These thresholds are based on health risk data and are used across UK primary care and specialist services:
- Underweight: BMI below 18.5 — may indicate nutritional concerns
- Healthy weight: BMI 18.5–24.9 — associated with lowest health risks
- Overweight: BMI 25.0–29.9 — increased risk of weight-related health conditions
- Obese (Class I): BMI 30.0–34.9 — significantly elevated health risks
- Severely obese (Class III): BMI 40.0+ — highest health risks, typically eligible for NHS specialist services
The NHS also recognises ethnicity-adjusted thresholds for people of South Asian, Chinese, Black African, and Black Caribbean heritage, who face higher cardiovascular and diabetes risks at lower BMI levels. Your GP can discuss which category applies to you and what it means for your individual health.
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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 Fat Percentage and Why It Matters
Body fat percentage measures the proportion of your weight that is fat tissue, separate from muscle, bone, and organs. Unlike the BMI formula, body fat assessment accounts for muscle mass — which is why an athletic person might have a "high" BMI but healthy body fat levels. The healthy body fat range varies by sex and age: women typically aim for 21–24% (fitness range), men for 14–17%. NHS weight management services increasingly use both BMI and waist circumference as a more complete assessment tool. Consider asking your GP about body composition assessment, especially if you're very muscular or have concerns about BMI alone.
- 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 BMI and body fat percentage?
BMI is a quick calculation using height and weight — it doesn't distinguish muscle from fat. Body fat percentage shows what proportion of your weight is actually fat tissue. Together, they give a fuller picture. An athlete might have high BMI but low body fat; someone sedentary might have normal BMI but excess body fat. The NHS uses both for a complete assessment.
Why do I need different BMI thresholds if I'm South Asian or Black British?
Research shows people of South Asian, Chinese, Black African, and Black Caribbean heritage develop type 2 diabetes and heart disease at lower BMI levels than white Europeans. The NHS recommends lower thresholds for these groups — healthy weight starts at BMI 18.5–22.9 instead of 24.9 — because health risk is higher at the same BMI.
What should I do if my BMI is in the overweight or obese range?
Your GP can assess your individual health and discuss options. The NHS weight management pathway starts with lifestyle support (tier 1), then primary care interventions (tier 2). If needed, you may be referred to specialist services (tier 3) where prescription medications like GLP-1 injections might be appropriate. Use our free BMI calculator and progress tracker to monitor changes alongside lifestyle changes.
Is BMI accurate for older adults or very muscular people?
BMI has limitations for both groups. Older adults lose muscle naturally, so BMI may overestimate health risk. Very muscular people (athletes, gym-goers) may have high BMI but low body fat. The NHS recommends also measuring waist circumference and considering overall fitness. Your GP can provide context for your individual situation.
How can I use a body fat calculator if I don't know my exact body fat percentage?
Most body fat assessment requires either lab testing (DEXA scan), bioelectrical impedance scales, or skin-fold measurements. MyWeightCheck's free BMI calculator provides your BMI instantly; discuss body fat testing with your GP if you need a formal assessment. Start with waist circumference, which the NHS uses alongside BMI for a practical health check.
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.