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 BMI And What Does It Mean?
BMI (Body Mass Index) is a measurement that relates your weight to your height to assess whether you're in a healthy weight range. It's calculated by dividing your weight in kilograms by your height in metres squared. The NHS uses BMI as a screening tool to identify whether adults may be at increased health risk due to excess weight. While BMI has limitations, it remains one of the most practical ways to understand whether your weight falls within a healthy range for your height. Understanding your BMI is a useful starting point for conversations with your GP about weight-related health.
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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 Ranges
The BMI formula is: weight (kg) ÷ height (m)². For example, if you weigh 70kg and are 1.75m tall, your BMI would be approximately 22.9. The NHS has established clear BMI categories to help you understand what your number means:
- Below 18.5: Underweight
- 18.5 to 24.9: Healthy weight (NHS recommendation)
- 25.0 to 29.9: Overweight
- 30.0 to 34.9: Obese (Class I)
- 35.0 to 39.9: Obese (Class II)
- 40.0+: Severely obese (Class III)
For adults of South Asian, Chinese, Black African, and Black Caribbean heritage, the NHS recommends lower thresholds: a healthy BMI is 18.5 to 22.9, and overweight begins at 23.0. Use our free BMI calculator above to find your personal BMI instantly and see where you fall within these NHS ranges.
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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 Limitations And What Else Matters
BMI limitations are significant: BMI does not distinguish between muscle and fat, so athletes or very muscular people may have a high BMI despite low body fat. BMI also doesn't account for where fat is stored—waist circumference is equally important. The NHS recommends measuring your waist: men should aim for under 94cm (37in), and women under 80cm (31.5in) to reduce health risks. BMI is also less accurate for elderly adults, who may have lost muscle mass, and during pregnancy. A holistic view combines BMI with waist measurement, fitness level, and overall health. Our free weight converter helps you understand your weight across different units (kg, stone, lbs), and your GP can provide personalised health advice beyond BMI alone.
- 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.
Is BMI the only thing that matters for weight health?
No. While BMI is a useful screening tool, it should be considered alongside waist circumference, overall fitness, diet, and general health. Some people with high BMI are very fit with low body fat, while others in the "healthy" BMI range may have high body fat. Your GP can assess your individual health picture beyond BMI.
Why does the NHS use different BMI thresholds for different ethnic groups?
Research shows that people of South Asian, Black African, Black Caribbean, and Chinese heritage face higher health risks at lower BMI levels. These groups have increased susceptibility to type 2 diabetes and cardiovascular disease at BMIs that would be considered "overweight" for other populations. The NHS adjusted thresholds to reflect this evidence.
What's the difference between overweight and obese according to BMI?
According to NHS categories, overweight is a BMI of 25.0 to 29.9, while obese begins at 30.0. Obese is further divided into Class I (30–34.9), Class II (35–39.9), and Class III (40+). These distinctions help healthcare professionals assess health risk level more precisely.
Can I have a healthy weight but an unhealthy waist circumference?
Yes. You could have a BMI in the "healthy" range but excess fat around your abdomen, which increases cardiovascular and metabolic risk. The NHS recommends measuring both BMI and waist circumference for a complete picture of health risk.
How is BMI used in NHS weight management services?
The NHS uses BMI to identify adults who may benefit from weight management support. Generally, adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions, may be referred to specialist tier 3 weight management services. Your GP can assess your individual situation.
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.