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 Compute Body Mass Index
Body mass index (BMI) is a simple measurement that uses your height and weight to estimate whether you're at a healthy weight range according to NHS guidelines. Computing your BMI takes just seconds using our free BMI calculator on this page — or you can learn the formula and calculate it manually. Understanding how to compute body mass index helps you assess your health risk, though it's important to know that BMI has limitations and doesn't account for muscle mass, bone density, or where fat is stored on your body.
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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.
Understanding Your BMI Calculator Results
When you compute body mass index using the calculator on this page, your result falls into one of five NHS categories. A healthy weight BMI is between 18.5 and 24.9 — this range is associated with lower health risks. BMI of 25.0 to 29.9 is overweight, while 30.0 and above is obese (further divided into Class I, II, and III based on severity).
For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds: healthy weight is 18.5 to 22.9, and overweight starts at 23.0. This is because these groups face higher cardiovascular and diabetes risk at lower BMI levels.
- Under 18.5: Underweight — consider speaking to your GP
- 18.5–24.9: Healthy weight range
- 25.0–29.9: Overweight — lifestyle changes may help
- 30.0+: Obese — GP support recommended
Remember that BMI is a screening tool, not a diagnostic measure. Your actual health depends on many factors including exercise level, diet quality, and family history.
Clinically BackedWeight Loss
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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 vs Body Fat Percentage and Body Composition
While BMI is useful for population health screening, your body fat percentage and overall body composition tell a more complete health story. Two people at the same weight and height can have very different amounts of muscle versus fat — BMI doesn't distinguish between them. A muscular person might have a high BMI but low body fat percentage, whilst someone sedentary might have a "healthy" BMI but poor lean body mass.
Waist-to-height ratio and waist circumference are also useful indicators. The NHS considers a waist-to-height ratio below 0.5 healthy for all adults. You can compute BMI quickly, but combining it with measurements like waist circumference and physical activity levels gives your GP a clearer picture of your health risk. If you want a deeper assessment of your body composition, ask your GP about options like DEXA scans or bioelectrical impedance analysis.
- 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 formula to compute body mass index?
The formula is: BMI = weight (kg) ÷ height (m)². For example, if you weigh 70kg and are 1.73m tall, your BMI is 70 ÷ (1.73 × 1.73) = 23.4. Use our free BMI calculator above to compute your BMI instantly without a UK pharmacy calculation — it handles the maths for you.
Is BMI accurate for everyone?
BMI is a useful screening tool but has limitations. It doesn't account for muscle mass, bone density, or where fat is stored. Athletes and very muscular people may have a high BMI but low body fat percentage. It's also less accurate in elderly adults, during pregnancy, and for children. Always discuss your personal health with your GP rather than relying on BMI alone.
Why do South Asian and Black African adults have lower BMI thresholds?
The NHS recommends lower BMI thresholds (healthy weight 18.5–22.9 instead of 18.5–24.9) for South Asian, Chinese, Black African, and Black Caribbean populations because research shows they have higher cardiovascular and type 2 diabetes risk at the same BMI levels as other groups. This reflects genuine health risk differences, not arbitrary categorisation.
Can I compute BMI if I'm pregnant?
BMI is less reliable during pregnancy because weight gain is expected and necessary for fetal development. Instead of using standard BMI categories, the NHS recommends discussing weight management with your midwife or GP individually. Use our weight converter to track your weight in stone and lbs if helpful, but consult your healthcare team for personalised guidance.
What's the difference between BMI and body fat percentage?
BMI uses height and weight only; body fat percentage measures the actual proportion of fat tissue versus muscle and bone. Someone with 25% body fat and someone with 35% body fat could have identical BMI. Body composition is a more accurate health indicator for individuals, but it requires specialist measurement or a body fat calculator rather than simple height and weight figures.
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.