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
How to Calculate Body Mass: The BMI Formula Explained
Body mass index (BMI) is calculated using a simple mathematical formula that divides your weight by your height squared. Whether you measure in kilograms and metres or prefer stone and pounds, understanding the body mass calculation formula helps you interpret your health data accurately. This guide explains both the metric and imperial versions, plus how your result fits into NHS weight categories and relates to overall body composition beyond just the numbers on the scale.
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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.
The Body Mass Calculation Formula: Metric and Imperial
The metric BMI formula is the most straightforward: BMI = weight (kg) ÷ height (m)². Simply divide your weight in kilograms by your height in metres, squared. For example, if you weigh 70 kg and are 1.75 metres tall, your BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.9.
The imperial approximation is: BMI = (weight in lbs × 703) ÷ height (inches)². If you prefer to work in stone and pounds, convert first: 70 kg ≈ 11 stone (154 lbs), at 5ft 9in (69 inches), BMI = (154 × 703) ÷ (69 × 69) ≈ 22.7.
Once calculated, the NHS categorises results: under 18.5 = underweight, 18.5–24.9 = healthy weight, 25.0–29.9 = overweight, 30.0+ = obese. Use our free BMI calculator above to compute your result instantly in either measurement system.
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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.
Beyond Body Mass: Why Body Composition and Limitations Matter
While the body mass calculation formula is useful, it has important limitations. BMI does not distinguish between muscle and fat—athletes may have high BMI despite low body fat percentage. For a fuller picture of health, consider body fat percentage, lean body mass, and waist circumference alongside BMI. The NHS recommends measuring waist circumference: over 94 cm (men) or 80 cm (women) signals increased health risk, regardless of BMI. South Asian, Chinese, Black African, and Black Caribbean adults face higher health risks at lower BMI thresholds, so ethnicity-adjusted categories apply. Your GP can assess your individual body composition and discuss what your BMI means for your overall health.
- 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 the difference between the metric and imperial BMI formulas?
The metric formula (BMI = kg ÷ m²) is simpler and used by the NHS. The imperial version (BMI = lbs × 703 ÷ inches²) gives the same result but requires more steps. Both produce identical outcomes; choose whichever measurements you're comfortable with. Our weight converter can help you switch between them.
Why do some people have high BMI but look lean?
Muscle is denser than fat, so muscular individuals (athletes, gym-goers) may have high BMI despite low body fat percentage. BMI alone cannot distinguish between muscle and fat tissue, which is why body composition analysis—using body fat calculators or scanning—provides a fuller health picture.
Does the body mass calculation formula work for children?
No. Children's BMI uses age and sex-specific centile charts, not adult categories. If you need to assess a child's healthy weight, speak to your GP or use the NHS Healthy Weight Calculator, which has separate tools for young people.
How often should I recalculate my BMI?
You can check BMI monthly or as needed, especially during weight loss or lifestyle changes. Regular tracking helps you spot trends. Our free progress tracker lets you log weight and BMI over time to see your journey visually.
Is BMI the only measure of health I should consider?
No. BMI is a screening tool only. Waist circumference, body fat percentage, blood pressure, blood sugar, and fitness level all matter. A conversation with your GP gives you a personalised health assessment beyond the formula alone.
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.