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 Figure Body Mass Index
Body mass index (BMI) is a simple measurement that relates your weight to your height, helping assess whether you're in a healthy weight range according to NHS guidelines. Calculating BMI takes just seconds using the free BMI calculator on this page, but understanding what your result means is equally important. This guide walks you through the calculation, how to interpret your score, and when BMI is most useful alongside other body composition measures like body fat percentage and waist circumference.
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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.
How To Interpret Your BMI Calculator Results
Once you've calculated your BMI using the calculator above, your result falls into one of five NHS categories. For most UK adults, a healthy BMI is between 18.5 and 24.9. A BMI of 25.0 to 29.9 is classified as overweight, while 30.0 and above is obese (with further subcategories for Class I, II, and III obesity). However, if you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds — a healthy range starts at 18.5 but extends only to 22.9, reflecting the higher cardiovascular and metabolic risks these groups face at higher BMI levels.
- BMI 18.5–24.9: Healthy weight (or 18.5–22.9 for South Asian, Chinese, Black African/Caribbean adults)
- BMI 25.0–29.9: Overweight
- BMI 30.0–34.9: Obese (Class I)
- BMI 35.0–39.9: Obese (Class II)
- BMI 40.0+: Severely obese (Class III)
Remember that your individual health depends on many factors beyond BMI alone, including your waist circumference, overall fitness level, and medical history. Your GP can provide personalised guidance on what your BMI means for your specific circumstances.
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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.
Body Mass Index vs Body Composition Metrics
Whilst BMI is a useful screening tool, it has important limitations. BMI does not distinguish between muscle and fat — an athlete with high muscle mass may have a high BMI but low body fat percentage and excellent health. This is where measuring body fat percentage and waist-to-height ratio become valuable. Body fat percentage directly measures the proportion of fat in your body, whilst a waist to height ratio below 0.5 indicates lower cardiovascular risk regardless of BMI. For most people, BMI is a good starting point, but if you're very muscular, elderly, or pregnant, these alternative metrics provide a more complete picture of your body composition and health status.
- 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 the BMI formula?
The metric formula is BMI = weight (kg) ÷ height (m)². For example, if you weigh 75kg and are 1.75m tall, your BMI = 75 ÷ (1.75)² = 24.5. The imperial version uses weight in pounds and height in inches. Use our free BMI calculator above to avoid a UK pharmacy calculations.
Why do South Asian and Black African adults have different BMI thresholds?
The NHS recommends lower BMI thresholds for these groups because research shows they face higher risks of type 2 diabetes and cardiovascular disease at lower BMI levels compared to white European adults. A BMI of 23 may carry equivalent health risk for a South Asian adult as a BMI of 27 for a white European adult.
Is BMI accurate for very muscular people?
No — BMI doesn't distinguish muscle from fat. Athletes and highly muscular individuals may have a high BMI but low body fat percentage and excellent health. In these cases, measuring actual body fat percentage or using waist-to-height ratio gives a more accurate picture of health risk.
What's the difference between BMI and body fat percentage?
BMI compares weight to height only. Body fat percentage measures the actual proportion of fat in your body. Someone can have the same BMI as another person but very different body fat percentages depending on their muscle mass. Both metrics together provide a clearer health picture than BMI alone.
Can I lose weight and still have the same BMI?
Yes — if you lose fat but gain muscle through exercise, your weight may stay the same, keeping BMI unchanged, but your body composition improves significantly. This is why tracking body fat percentage and how clothes fit can be more meaningful than BMI alone, especially during weight loss with exercise.
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.