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
How To Work Out Body Mass
Body mass, more accurately called Body Mass Index (BMI), is calculated using your weight and height. The standard formula divides your weight in kilograms by your height in metres squared. BMI is a useful screening tool used by the NHS to categorise whether someone is at a healthy weight, though it has important limitations. Understanding how to calculate your own body mass can help you assess your personal health context alongside other measurements like waist circumference and body fat percentage.
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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 Calculate BMI Using The Formula
The BMI formula is simple: divide your weight in kilograms by your height in metres, then square the height value. For example, if you weigh 70kg and are 1.75m tall, you would calculate: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.9 BMI.
Once calculated, the NHS categorises your BMI as follows: below 18.5 is underweight, 18.5–24.9 is a healthy weight, 25–29.9 is overweight, 30–34.9 is obese (Class I), 35–39.9 is obese (Class II), and 40+ is severely obese (Class III).
For adults 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. Our free BMI calculator above will work this out instantly for you — simply enter your weight and height in either metric or imperial units, and it will show your personal BMI category.
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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.
BMI Limitations And Body Composition
BMI is a screening tool but does not distinguish between muscle mass and fat mass. An athlete with significant lean body mass may have a high BMI but low body fat percentage, meaning they are actually very healthy. Similarly, someone with a normal BMI but high body fat could be at greater health risk.
For a more complete picture, consider your body composition alongside BMI. Waist-to-height ratio is also valuable: a healthy ratio is below 0.5 (waist circumference ÷ height in the same units). Your GP can discuss whether your body mass is healthy in context of your overall fitness, genetics, and medical history using our free weight converter to compare your weight in stone, kilograms, or pounds.
- 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 fat percentage?
Body mass (BMI) measures total weight relative to height, whilst body fat percentage measures the proportion of your weight that is fat tissue. BMI does not distinguish between fat and muscle, so two people with the same BMI can have very different body compositions. Body fat percentage provides more detail about your actual body composition.
Can you have a healthy BMI but high body fat?
Yes. Someone can have a normal BMI (18.5–24.9) but still have excess body fat if they have low muscle mass. This is sometimes called "normal weight obesity". Conversely, someone very muscular may have a high BMI but low body fat. This is why body composition analysis is valuable alongside BMI.
Are BMI thresholds different for different ethnicities?
Yes. The NHS recommends lower BMI thresholds for people of South Asian, Chinese, Black African, or Black Caribbean heritage because these groups face higher cardiovascular and diabetes risk at lower BMI levels. For these groups, healthy weight is 18.5–22.9 (not 18.5–24.9), and overweight begins at 23.
What is a healthy body fat percentage for adults in the UK?
For women, healthy body fat is 21–31%; for men, it is 18–24%. Body fat percentage is more accurate than BMI alone because it accounts for muscle mass. However, body fat percentage can only be measured accurately using professional methods like DEXA scans or bioelectrical impedance analysis.
How does waist-to-height ratio relate to body mass?
Waist-to-height ratio measures central fat distribution, which is a strong predictor of cardiovascular disease risk. A healthy ratio is below 0.5 — calculate it by dividing your waist circumference by your height (both in the same units). This complements BMI by showing where excess fat is located.
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.