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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Understanding the BMI Scale in the UK
The BMI scale is a simple measurement tool used by the NHS to screen for weight-related health risks in adults. Your BMI (body mass index) is calculated by dividing your weight in kilograms by your height in metres squared. The NHS uses standardised BMI categories to help you understand whether your weight falls into a healthy range for your height. Our free BMI calculator above gives you your personal BMI instantly, and this guide explains what your result means according to NHS guidance.
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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 Weight Ranges
The NHS BMI scale divides adults into four main weight categories based on their body mass index score:
- Healthy weight: BMI 18.5 to 24.9 — associated with lower health risks
- Overweight: BMI 25.0 to 29.9 — increased risk of weight-related conditions
- Obese (Class I): BMI 30.0 to 34.9 — higher health risks
- Obese (Class II): BMI 35.0 to 39.9 — significantly higher risks
- Severely obese (Class III): BMI 40.0 and above — substantial health risks requiring medical support
These thresholds apply to most UK adults. However, the NHS recommends lower BMI thresholds for people of South Asian, Chinese, Black African, and Black Caribbean heritage, as these groups face higher risk of type 2 diabetes and cardiovascular disease at lower BMI levels. For these populations, a healthy BMI range starts at 18.5 but tops out at 22.9, with overweight beginning at 23.0 and obesity at 27.5.
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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 Support and When to Seek Help
If your BMI range indicates overweight or obesity, the NHS offers a structured weight management pathway through your GP. NICE guidance recommends lifestyle changes as the first step—such as reduced-calorie diet and increased physical activity. Your GP may refer you to tier 3 specialist weight management services, where you may be assessed for prescription weight loss medications if appropriate. The NHS also emphasises that BMI has limitations: it doesn't distinguish between muscle and fat, so athletes or very muscular individuals may have a higher BMI despite low body fat. Waist measurement and overall health context matter too. Book an appointment with your GP if you're concerned about your weight or want personalised guidance.
- 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 according to the NHS?
The NHS defines a healthy BMI as 18.5 to 24.9 for most UK adults. However, if you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends a healthy range of 18.5 to 22.9. Use our free BMI calculator to find your personal BMI and see where you fall on the NHS scale.
At what BMI does the NHS consider someone overweight?
The NHS BMI scale classifies overweight as 25.0 to 29.9. If your BMI falls in this range, you have an increased risk of weight-related health conditions like type 2 diabetes and high blood pressure. Your GP can discuss lifestyle changes or refer you to specialist support if needed.
What is the difference between obese and severely obese on the BMI scale?
Obese Class I is BMI 30.0 to 34.9, and Obese Class II is 35.0 to 39.9. Severely obese (Class III) is BMI 40.0 and above. Higher classifications indicate progressively greater health risks. The NHS may offer more intensive weight management support at these levels, including consideration of specialist treatment.
Does the NHS BMI scale apply to everyone equally?
No. The standard BMI categories apply to most adults, but the NHS recommends lower thresholds for people of South Asian, Chinese, Black African, and Black Caribbean heritage due to higher health risks at lower BMI levels. BMI also doesn't account for muscle mass, so very muscular individuals may have a higher BMI but remain healthy.
When should I contact my GP about my BMI?
If your BMI is 25 or above, or if you have concerns about your weight and health, book an appointment with your GP. They can assess your individual situation, discuss weight management options, and refer you to specialist services if appropriate. The NHS Weight Loss Plan and tier 3 specialist services are available in many areas.
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.