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 Does BMI Mean? Definition and UK Explained
BMI stands for Body Mass Index — a measurement of body fat based on your height and weight. It's a screening tool used by the NHS and healthcare professionals across the UK to assess whether an adult is at a healthy weight. BMI helps identify potential health risks associated with being underweight, overweight, or obese, though it's just one part of a complete health picture. Understanding your BMI is a useful starting point for conversations with your GP about weight-related health.
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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 and UK Categories
The BMI formula is straightforward: divide your weight in kilograms by your height in metres squared (BMI = weight ÷ height²). For example, if you weigh 70kg and are 1.75m tall, your BMI is 70 ÷ (1.75)² = 22.9.
The NHS BMI range divides adults into these categories:
- Underweight: BMI below 18.5
- Healthy weight: BMI 18.5–24.9
- Overweight: BMI 25.0–29.9
- Obese (Class I): BMI 30.0–34.9
- Obese (Class II): BMI 35.0–39.9
- Severely obese (Class III): BMI 40.0+
For adults of South Asian, Chinese, Black African, or Black Caribbean backgrounds, the NHS recommends lower healthy BMI thresholds — typically 18.5–22.9 — because these populations face higher health risks at the same BMI levels. Use our free BMI calculator above to find your personal BMI instantly.
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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 What Else Matters
BMI is a useful screening tool, but it has important BMI limitations. It doesn't distinguish between muscle and fat — so athletes or very muscular individuals may have a high BMI despite being fit. BMI also ignores where fat is stored; abdominal fat carries greater health risk than fat stored elsewhere. It's less accurate for older adults (who naturally lose muscle), during pregnancy, or for very tall or very short people.
Complementary measures are equally important: waist circumference (men over 94cm, women over 80cm indicates increased risk), waist-to-height ratio (healthy if below 0.5), and body composition analysis. Your GP can assess these alongside your BMI to give you a complete health picture and personalised advice about your individual weight and health.
- 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 a healthy BMI range in the UK?
According to NHS guidelines, a healthy BMI for most UK adults is 18.5–24.9. However, for people of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends a lower healthy range of 18.5–22.9 due to higher cardiovascular and diabetes risk at the same BMI levels.
Can I calculate my own BMI at home?
Yes. Divide your weight in kg by your height in metres squared (BMI = weight ÷ height²). For example, 70kg ÷ (1.75)² = 22.9. You can also use our free BMI calculator above for gradual improvement in both metric and imperial measurements.
Why do South Asian and Black African adults have different BMI categories?
Research shows these populations develop type 2 diabetes, heart disease, and metabolic complications at lower BMI levels than European populations. The NHS recognises this health inequality and recommends earlier intervention at BMI 23.0–27.4 (overweight) rather than the standard 25.0–29.9 threshold.
Does a high BMI always mean I'm unhealthy?
Not necessarily. BMI doesn't account for muscle mass, bone density, or fitness level. Athletes may have high BMI but low body fat. However, BMI is a useful screening tool. If your BMI is elevated, your GP can assess your overall health, waist circumference, and other factors to determine actual health risk.
What should I do if my BMI is in the overweight range?
Speak with your GP about your individual health picture. They can advise on diet, physical activity, and whether any medical interventions (like weight loss medication or specialist services) might be appropriate. The NHS Weight Loss Plan and our weight converter and BMI calculator can help you track progress.
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.