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 Is A BMI — And Why Does The NHS Use It?
BMI stands for Body Mass Index — a simple measurement that helps you and your GP understand whether your weight is in a healthy range for your height. The NHS uses BMI as a screening tool to assess health risk, though it's important to know that BMI has limitations and should always be considered alongside other health factors like fitness, muscle mass, and waist circumference. This guide explains what BMI is, how it's calculated, and what the NHS BMI categories mean for your health in the UK context.
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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: divide your weight in kilograms by your height in metres squared (kg ÷ m²). The NHS BMI range categories are clear and evidence-based:
- Under 18.5: Underweight — may indicate insufficient nutrition
- 18.5 to 24.9: Healthy weight — the healthy BMI range recommended by NHS
- 25.0 to 29.9: Overweight — increased health risks begin here
- 30.0 to 34.9: Obese (Class I) — significant health risks
- 35.0 to 39.9: Obese (Class II) — greater health risks
- 40.0 and above: Severely obese (Class III) — highest health risks
However, if you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower BMI categories UK thresholds because these groups face higher risk of type 2 diabetes and heart disease at lower BMI levels. For these populations, healthy weight is 18.5–22.9, and overweight begins at 23.0. You can calculate your personal BMI instantly using our free BMI calculator above to see exactly where you fall.
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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.
NHS Weight Management — When To Seek Help
The NHS recognises that BMI limitations exist — BMI doesn't account for muscle mass, bone density, or fat distribution — but it remains a useful starting point. If your BMI falls outside the healthy range, the NHS recommends speaking to your GP, who can assess your individual health picture. The NHS offers a structured weight management pathway: Tier 1 includes self-help and digital tools (like our BMI calculator and weight converter), Tier 2 involves primary care behaviour-change programmes, and Tier 3 provides specialist multidisciplinary services including prescription medication if appropriate. Your GP can advise which tier suits you and discuss sustainable weight management options tailored to your needs.
- 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 BMI formula used by the NHS?
The NHS BMI formula is: weight in kilograms divided by height in metres squared (kg ÷ m²). For example, an adult weighing 70kg and 1.75m tall would have a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which falls in the healthy weight range.
What is a healthy BMI according to NHS guidelines?
According to NHS guidance, a healthy BMI range is 18.5 to 24.9 for most UK adults. However, for people of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends a lower threshold of 18.5 to 22.9 due to increased health risks at higher BMI levels in these populations.
What does overweight BMI threshold mean in the UK?
The overweight BMI threshold in the UK is 25.0 to 29.9. If your BMI falls in this range, the NHS advises speaking to your GP about lifestyle changes and weight management strategies to reduce health risks associated with excess weight.
What are the main BMI limitations I should know about?
The key BMI limitations are: it doesn't distinguish muscle from fat (athletes may have high BMI but low body fat), it ignores where fat is stored (waist circumference matters), and it's less accurate for elderly adults, pregnant women, and very muscular individuals. Always discuss results with your GP.
How do I find out my BMI and what it means for my health?
Use our free BMI calculator on this page to find your BMI instantly. It will show you which NHS category you fall into and provide context. For personalised health advice based on your BMI and individual circumstances, contact your GP who can refer you to NHS weight management services if needed.
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.