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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Body Mass Indicator UK — How to Interpret Your Results
Body mass indicator (BMI) is a simple screening tool that assesses your weight in relation to your height, helping identify potential health risks. However, it's only one part of understanding your overall body composition. This guide explains how to interpret your results from our free calculator above, what the ranges mean for UK adults, and why body composition matters beyond a single number. Whether you're tracking your health or considering weight management, understanding these metrics helps you make informed decisions with your GP.
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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.
How to Interpret Your Body Mass Indicator Results
Your body mass indicator (BMI) falls into one of five categories according to NHS guidelines. Use our free BMI calculator above to find your exact number, then match it to the reference range below.
- Below 18.5: Underweight — may indicate insufficient nutrition or other health concerns
- 18.5–24.9: Healthy weight — associated with lowest health risk for most adults
- 25.0–29.9: Overweight — increased risk of weight-related health conditions
- 30.0–34.9: Obese (Class I) — increased health risks warrant GP discussion
- 35.0–39.9: Obese (Class II) — higher health risk; medical support may be beneficial
- 40.0+: Severely obese (Class III) — significant health risks; specialist support recommended
If you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds: healthy weight is 18.5–22.9, and overweight begins at 23.0. These adjustments reflect higher cardiovascular disease risk in these populations at lower BMI levels.
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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 Body Mass Indicator — Which Matters More?
Your body mass indicator is useful, but it doesn't distinguish between muscle and fat. Two people with identical BMI can have vastly different body fat percentages and health profiles. Body composition — the ratio of fat to lean body mass — is often more informative. Waist circumference is also important: men with waist over 94cm (37in) face increased risk, and over 102cm (40in) face high risk. Women with waist over 80cm (31.5in) have increased risk, and over 88cm (34.5in) have high risk.
For most people, BMI is a practical starting point. Athletes and very muscular individuals may have high BMI but low fat percentage. Conversely, sedentary individuals may have normal BMI but poor body composition. Use our free weight converter and BMI calculator together to get a fuller picture of your health profile, and discuss any concerns with your GP.
- 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 mass indicator and body fat percentage?
BMI is a ratio of weight to height and doesn't account for muscle mass. Body fat percentage measures the actual percentage of your weight that is fat tissue. Someone with high muscle mass might have a high BMI but low body fat percentage. Both metrics are useful — BMI is simple screening, body fat percentage is more detailed body composition assessment.
Is my BMI accurate if I'm very muscular or athletic?
No. BMI can overestimate health risk for athletes and muscular individuals because it doesn't distinguish muscle from fat. If you're very active with significant muscle mass, discuss your results with your GP rather than relying on BMI alone. Waist circumference and body composition assessment are more accurate for you.
Why do people of South Asian heritage have different BMI thresholds in the UK?
Research shows South Asian, Chinese, Black African, and Black Caribbean populations have higher risk of type 2 diabetes and heart disease at lower BMI levels than white Europeans. The NHS therefore recommends lower thresholds for these groups — healthy weight starts at BMI 18.5–22.9 rather than 18.5–24.9. This reflects genetic and metabolic differences, not body composition differences.
Can I have a healthy BMI but poor body composition?
Yes. Someone can fall into the "healthy weight" BMI range but have high body fat and low lean muscle mass — sometimes called "skinny fat." This is why combining BMI assessment with waist measurement and regular physical activity is important. Use our free BMI calculator alongside waist measurement guidance from the NHS for a more complete picture.
How often should I check my body mass indicator and body composition?
Monthly checks are reasonable if you're actively managing weight or health. Weekly checks can fluctuate due to water retention and are less meaningful. Quarterly or annual checks are fine for general health monitoring. Track changes over time using our free progress tracker rather than focusing on single measurements.
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.