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 Work Your BMI Out
Working out your BMI (Body Mass Index) is straightforward using a simple formula based on your weight and height. The NHS uses BMI as a screening tool to help assess whether an adult is at a healthy weight, though it's important to understand that BMI has limitations and doesn't account for muscle mass or where fat is stored on your body. In this guide, we'll show you exactly how to calculate your BMI, what the NHS categories mean, and when you might want to discuss your results with your GP. You can also use our free BMI calculator above to get your result instantly.
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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.
The BMI Formula and How to Calculate It
BMI is calculated using a simple BMI formula that divides your weight in kilograms by your height in metres squared. The metric formula is: BMI = weight (kg) ÷ height (m)². Here's how to work it out step by step:
- Write down your weight in kilograms (if you know it in stone and pounds, convert to kg first using our free weight converter)
- Write down your height in metres and convert to decimal (for example, 1.73m)
- Multiply your height by itself (1.73 × 1.73 = 2.99)
- Divide your weight by this number — this is your BMI
Example: If you weigh 70kg and are 1.73m tall: 70 ÷ 2.99 = 23.4 BMI. According to NHS guidelines, this falls in the healthy weight BMI range of 18.5 to 24.9. The NHS also recommends that adults of South Asian, Chinese, Black African, or Black Caribbean heritage use a slightly lower healthy BMI threshold of 18.5 to 22.9, as these groups face higher health risks at the same BMI.
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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 BMI Categories and What They Mean
Once you've calculated your BMI, the NHS provides clear BMI categories UK to interpret your result. A BMI of 18.5–24.9 is considered a healthy weight, 25.0–29.9 is overweight, 30.0–34.9 is obese (Class I), 35.0–39.9 is obese (Class II), and 40.0 and above is severely obese (Class III). Understanding where you fall helps you and your GP assess your health risk. The NHS weight management pathway recommends that if your BMI is 30 or above, or 27 or above with weight-related health conditions like type 2 diabetes or high blood pressure, it's worth discussing with your GP. They can offer support through NHS weight management services, including tier 3 specialist services if needed. Remember, BMI is a useful starting point but has BMI limitations — it doesn't measure body fat directly or account for muscle mass, so your GP will consider your full health picture.
- 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's the difference between my BMI and my actual body fat percentage?
BMI is a weight-to-height ratio and doesn't measure body fat directly. Someone with significant muscle mass may have a high BMI but low body fat. Body fat percentage is measured differently, using methods like DEXA scans or bioelectrical impedance. Your GP can advise on which measurement is most useful for your health.
Is the BMI formula the same for men and women?
Yes, the BMI formula is exactly the same for both men and women. However, the healthy weight ranges and health risks associated with the same BMI can differ slightly between sexes. The NHS uses the same BMI categories for all adults, but individual health context matters.
Why does the NHS recommend a lower BMI for people of South Asian or Black African heritage?
Research shows that people of South Asian, Chinese, Black African, and Black Caribbean heritage have a higher risk of type 2 diabetes and heart disease at lower BMI levels compared to other populations. The NHS therefore recommends using a lower healthy BMI threshold (18.5–22.9) for these groups to better identify those at increased health risk.
What should I do if my BMI calculation suggests I'm overweight or obese?
Having a BMI in the overweight or obese range doesn't mean you're unhealthy, but it may suggest discussing your health with your GP. They can assess your overall health, check for weight-related conditions, and discuss support options including lifestyle changes, NHS weight management services, or specialist referral if appropriate.
Can I use the BMI categories from other countries, or should I use the NHS categories?
Use the NHS categories — they're specifically designed for UK adults and align with NICE guidance. Some countries use slightly different thresholds, so it's important to use the UK standard. Our BMI calculator on this page uses NHS categories, ensuring your result is accurate for the UK health system.
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.