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 A BMI Mean?
BMI stands for body mass index — a simple measure that uses your height and weight to estimate whether you're in a healthy weight range. The NHS uses BMI as a screening tool to check if your weight may affect your health. In the UK, healthy BMI is between 18.5 and 24.9. Understanding what your BMI means helps you make informed decisions about your health, though it's important to know that BMI is just one part of the picture. Use our free BMI calculator above to check your personal BMI instantly and see where you fall within the NHS categories.
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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.
BMI Formula and UK Health Categories
BMI is calculated using a simple formula: weight (kg) divided by height in metres, squared. For example, if you weigh 70kg and are 1.75m tall, your BMI would be approximately 22.9 — which falls in the healthy weight range.
The NHS uses these BMI categories to guide health decisions:
- Below 18.5: Underweight
- 18.5 to 24.9: Healthy weight
- 25.0 to 29.9: Overweight
- 30.0 to 34.9: Obese (Class I)
- 35.0 to 39.9: Obese (Class II)
- 40.0 and above: Severely obese (Class III)
If you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower BMI thresholds — your healthy weight range may start at 18.5 and extend to 22.9. This is because these groups face higher health risks at lower BMI levels. Your GP can explain what your specific BMI range means for your health.
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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.
Important BMI Limitations and What Else Matters
BMI doesn't tell the whole story. It cannot distinguish between muscle and fat — so athletes with high muscle mass may have a high BMI while having very little body fat. BMI also doesn't account for where fat is stored, which matters for health risk: fat around your abdomen (belly fat) is linked to higher cardiovascular and diabetes risk than fat elsewhere.
The NHS recommends also measuring your waist circumference to get a fuller picture. For men, a waist over 94cm (37 inches) indicates increased risk; over 102cm (40 inches) signals high risk. For women, increased risk starts at 80cm (31.5 inches) and high risk at 88cm (34.5 inches). Age, genetics, and medical conditions also influence your health — your BMI alone doesn't determine your overall wellness. Always speak with your GP for personalised health 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's a good BMI range for a UK adult?
According to NHS guidelines, a healthy BMI is between 18.5 and 24.9 for most adults. If you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the healthy range is 18.5 to 22.9. Calculate your BMI using our free calculator above to see where you fall and what it means for your health.
Why is BMI different for different ethnic backgrounds?
Research shows that people of South Asian, Chinese, Black African, and Black Caribbean descent face higher risks of type 2 diabetes and heart disease at lower BMI levels compared to other groups. The NHS adjusted the thresholds for these populations to better identify health risk earlier and enable earlier intervention.
Does a high BMI always mean I'm unhealthy?
Not necessarily. BMI doesn't distinguish between muscle and fat, so athletes or very muscular individuals may have a high BMI while being perfectly healthy. However, if you have excess body fat at a high BMI, your health risk increases. Your GP can assess your overall health picture, including waist measurement and medical history, for a complete view.
Can I use BMI to check if I'm overweight?
Yes — BMI is useful for screening. A BMI of 25.0 to 29.9 falls in the overweight category according to NHS guidelines. The overweight BMI threshold indicates you may benefit from lifestyle changes. Use our weight converter and BMI calculator together to understand your current weight status and healthy weight range for your height.
What should I do if my BMI is outside the healthy range?
Start by speaking with your GP, who can assess your individual health and advise on next steps. Even a modest 5–10% weight reduction can improve health outcomes. Your GP may suggest lifestyle changes, weight loss support programmes, or other interventions depending on your specific circumstances and health needs.
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.