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 Determine?
BMI (body mass index) is a screening tool that helps determine whether your weight falls into a healthy range for your height. The NHS uses BMI to assess health risk and identify whether someone may need support with weight management. However, BMI is just one part of the picture — it doesn't measure body fat directly or account for muscle mass, bone density, or where fat is stored on your body. Your GP can use BMI alongside other measurements to give you personalised health advice. Use our free BMI calculator above to check your personal BMI instantly and see where you fall on the NHS scale.
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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.
NHS BMI Categories and What They Mean
The NHS uses a standard BMI formula — weight in kilograms divided by height in metres squared — to categorise adults into health risk groups. These NHS BMI categories are: underweight (below 18.5), healthy weight (18.5–24.9), overweight (25.0–29.9), obese class I (30.0–34.9), obese class II (35.0–39.9), and severely obese (40.0+). A healthy BMI of 18.5–24.9 suggests lower health risk, while higher ranges indicate increased risk of weight-related conditions like type 2 diabetes, heart disease, and high blood pressure. Importantly, for adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower BMI thresholds — a healthy range of 18.5–22.9 — because these populations face higher health risks at lower BMI levels. Your GP can explain what your individual BMI means for your health and whether any action is needed.
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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.
What BMI Doesn't Determine — Important Limitations
Despite its usefulness, BMI has significant BMI limitations. It cannot distinguish between muscle and fat — athletes often have high BMI but low body fat. BMI also ignores where fat is stored; excess fat around the waist (apple shape) poses greater health risk than fat around the hips (pear shape). The NHS recommends measuring waist circumference alongside BMI for a fuller picture: men should aim below 94cm, and women below 80cm. Older adults, pregnant women, and children have different health thresholds that adult BMI categories don't capture. That's why your GP uses BMI as a screening tool, not a definitive diagnosis. Our free weight converter helps you track your weight in whatever units you prefer whilst you work towards your health goals.
- 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 NHS healthy BMI range for UK adults?
The NHS defines a healthy BMI as 18.5–24.9 for most UK adults. However, for South Asian, Chinese, Black African, or Black Caribbean adults, the NHS recommends a lower healthy range of 18.5–22.9 due to higher health risks at lower BMI. Your GP can advise on what range is appropriate for you.
Can BMI determine if I'm overweight?
Yes, BMI helps screen for overweight status. A BMI of 25–29.9 is classified as overweight by the NHS. However, BMI alone doesn't account for muscle mass or fat distribution, so your GP may use additional measurements like waist circumference to give you complete health advice.
Does a high BMI always mean poor health?
A high BMI increases health risk, but it's not the only factor. Fitness level, diet quality, blood pressure, cholesterol, and family history all matter. Some people with higher BMI are metabolically healthy. Always discuss your personal health picture with your GP rather than relying on BMI alone.
What should I do if my BMI falls in the overweight category?
If your BMI is 25–29.9, talk to your GP about whether lifestyle changes would help. The NHS recommends a balanced diet, regular physical activity, and aiming for gradual weight loss of 0.5–1kg per week if needed. Your GP may refer you to weight management support services.
Is BMI different for children?
Yes, children and teenagers use age and sex-specific BMI centile charts — adult categories don't apply. The NHS uses UK90 growth reference charts to assess children's weight. If you're concerned about your child's weight, speak to your GP or health visitor for guidance.
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.