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 Compute Your Body Mass Index
Body mass index computation is a straightforward calculation that uses your weight and height to estimate whether you're at a healthy weight. The NHS uses BMI as a screening tool to assess health risk in UK adults. Your BMI is calculated by dividing your weight in kilograms by your height in metres squared. Whether you measure in stone, pounds, or kilograms, our free BMI calculator above will compute your result instantly and show you where you fall on the NHS healthy weight scale. Understanding how to compute and interpret your BMI is an important first step in assessing your overall health.
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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.
Interpreting Your Body Mass Index Results
Once you've computed your BMI, the next step is understanding what your result means. The NHS BMI categories are:
- Below 18.5: Underweight
- 18.5–24.9: Healthy weight
- 25.0–29.9: Overweight
- 30.0–34.9: Obese (Class I)
- 35.0–39.9: Obese (Class II)
- 40.0+: Severely obese (Class III)
If you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower BMI thresholds: healthy weight is 18.5–22.9, and overweight begins at 23.0. This is because these populations face higher cardiovascular and diabetes risk at the same BMI as other groups. Your computed BMI result tells you which category you fall into, but it's important to remember that BMI doesn't distinguish between muscle and fat—athletes may have a high BMI but low body fat percentage. Consider your overall body composition, not BMI alone.
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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 Index vs Body Composition Metrics
While BMI computation is useful, it has limitations. Body fat percentage and lean body mass provide a more complete picture of your health than BMI alone. Body composition analysis accounts for the difference between fat mass and muscle mass, which BMI ignores. For example, someone with significant muscle mass may compute a high BMI despite having a healthy body fat percentage. Waist circumference is another valuable metric: men should aim for under 94cm (37 inches), and women under 80cm (31.5 inches). Together, BMI computation, body fat percentage, and waist measurement give a fuller assessment of health risk than any single metric. Use our BMI calculator to find your BMI, and discuss your results with your GP for personalised health advice.
- 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 exact formula for computing body mass index?
The formula is: BMI = weight (kg) ÷ height (m)². For example, if you weigh 70kg and are 1.75m tall, your BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.9. In imperial terms, the formula is: BMI = (weight in lbs × 703) ÷ height (inches)². Use our calculator above to compute this instantly.
Why are BMI thresholds lower for South Asian and Black African adults in the UK?
Research shows that South Asian, Chinese, Black African, and Black Caribbean populations develop type 2 diabetes and heart disease at lower BMI levels than other groups. The NHS therefore recommends lower thresholds: healthy weight is 18.5–22.9 instead of 18.5–24.9. Your GP can advise you on thresholds that apply to your heritage.
Is BMI computation accurate for muscular people or athletes?
No. BMI doesn't differentiate between muscle and fat, so muscular individuals or athletes may compute a high BMI despite having low body fat. If you're very muscular, discuss your results with your GP and consider body composition analysis (body fat percentage) alongside your BMI.
How does body fat percentage differ from computed BMI?
BMI uses height and weight only. Body fat percentage measures the proportion of your body that is fat versus lean tissue (muscle, bone, organs). Someone can compute the same BMI as another person but have very different body fat levels. Body composition analysis is more detailed but requires specialist equipment.
What should I do if my computed BMI falls in the overweight or obese range?
First, discuss your result with your GP. They can assess your individual health risk, check for weight-related conditions, and recommend lifestyle changes or treatment options if appropriate. The NHS Weight Loss Plan and specialist weight management services are available to eligible adults. Your GP can guide you toward the right support for your situation.
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.