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 BMI Mean?
BMI stands for Body Mass Index — a simple measurement of body weight relative to height. It's a screening tool that helps determine whether an adult's weight falls into a healthy range according to NHS guidelines. BMI isn't a perfect measure of health, but it gives a quick indication of whether excess weight might increase health risks. Your GP often uses BMI as a starting point when discussing weight and health. Use our free BMI calculator above to find your personal BMI instantly, then read below to understand what your result means.
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Calculate Your BMI Instantly
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 NHS Categories
BMI is calculated using this simple formula: weight in kilograms divided by height in metres squared (kg ÷ m²). For example, someone weighing 70kg and 1.73m tall would have a BMI of 23.4.
The NHS BMI range divides adults into four categories:
- Underweight: BMI below 18.5
- Healthy weight: BMI 18.5 to 24.9
- Overweight: BMI 25.0 to 29.9
- Obese: BMI 30.0 and above
Within the obese category, the NHS further breaks this down into Class I (30–34.9), Class II (35–39.9), and Class III (40+). If you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds — your healthy BMI range starts at 18.5 but extends only to 22.9, reflecting higher health risks at lower weights for these populations.
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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.
BMI Limitations and What Else Matters
BMI has important limitations you should know about. It doesn't distinguish between muscle and fat — an athlete with significant muscle mass might have a high BMI but low body fat. BMI also ignores where fat is stored; excess weight around the stomach poses greater health risks than weight distributed elsewhere. For this reason, your GP may also measure your waist circumference. A healthy waist measurement is under 94cm (37 inches) for men and 80cm (31.5 inches) for women. Use our free weight converter to track your weight in the units you prefer, and remember that BMI is just one part of the picture — overall fitness, diet quality, blood pressure, and cholesterol matter too.
- 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 a BMI of 25 overweight?
Yes, according to NHS guidelines, a BMI between 25.0 and 29.9 is classified as overweight. This doesn't automatically mean you're unhealthy, but it suggests your weight is above the healthy range for your height. Your GP can provide personalised advice about whether your weight poses any health risks.
What BMI is considered obese in the UK?
The NHS defines obesity as a BMI of 30.0 and above. This is further subdivided into Class I (30–34.9), Class II (35–39.9), and Class III (40+). A higher BMI typically indicates increased risk of weight-related health conditions like type 2 diabetes and heart disease.
Can BMI be inaccurate?
Yes, BMI has limitations. It doesn't account for muscle mass, bone density, or fat distribution. Athletes and very muscular people may have high BMI but low body fat. Conversely, BMI doesn't reflect fitness level or overall health. Your GP may recommend additional measurements like waist circumference for a fuller picture.
What is a healthy BMI for women and men?
The healthy BMI range is identical for both men and women: 18.5 to 24.9 according to NHS guidelines. However, for South Asian, Chinese, Black African, or Black Caribbean adults, the healthy upper limit is 22.9. Individual health needs vary, so discuss your specific healthy weight range with your GP.
How do I calculate my BMI?
Divide your weight in kilograms by your height in metres squared. For example, 70kg ÷ (1.73)² = 23.4 BMI. Our free BMI calculator above does this instantly — just enter your height and weight to see your result and NHS category.
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.