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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Formula For Body Mass — How To Calculate Your BMI
The formula for body mass, commonly known as BMI (Body Mass Index), is a simple calculation that divides your weight by your height squared. In the UK, we typically use the metric version: weight in kilograms divided by height in metres squared. It's a quick screening tool used by the NHS to assess whether your weight falls into a healthy range for your height. However, BMI has limitations — it doesn't distinguish between muscle and fat, so it's best used alongside other measurements like waist circumference and body fat percentage.
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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.
How To Calculate Your BMI Using The Formula
The metric BMI formula is: BMI = weight (kg) ÷ height (m)²
For example, if you weigh 70kg and are 1.73 metres tall: BMI = 70 ÷ (1.73 × 1.73) = 70 ÷ 2.99 = 23.4
The imperial approximation is: BMI = (weight in lbs × 703) ÷ height (inches)²
Once you've calculated your BMI, compare it to the NHS BMI categories:
- Below 18.5 = Underweight
- 18.5–24.9 = Healthy weight
- 25.0–29.9 = Overweight
- 30.0–34.9 = Obese (Class I)
- 35.0–39.9 = Obese (Class II)
- 40.0+ = Severely obese (Class III)
If you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds — healthy weight is 18.5–22.9. Use our free BMI calculator above to find your gradual improvement without a UK pharmacy calculation.
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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.
Understanding Body Composition Beyond The BMI Formula
Whilst the BMI formula is useful, it has important limitations. Body composition — the ratio of fat to lean muscle mass — tells a more complete health story than BMI alone. Someone with high muscle mass (like an athlete) may have a high BMI but low body fat percentage, whilst BMI misses this distinction. Lean body mass and body fat percentage provide more nuanced health insights. The NHS recommends checking your waist circumference alongside BMI: men should aim for under 94cm, women under 80cm. For a fuller picture of your health, discuss your body composition with your GP, or use our free BMI calculator and weight converter to track your measurements over time.
- 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 in simple terms?
BMI = weight (kg) ÷ height (m)². It's your weight divided by your height squared. For example, 70kg at 1.73m tall gives a BMI of 23.4. You can use our free BMI calculator on this page to calculate it instantly without doing the maths yourself.
Is the BMI formula accurate for everyone?
BMI is a useful screening tool but has limitations. It doesn't distinguish between muscle and fat, so athletes with high muscle mass may have a high BMI despite low body fat. It's also less accurate for older adults, very tall or short people, and during pregnancy. Always discuss your personal results with your GP.
What BMI range is healthy according to the NHS?
The NHS defines healthy weight as a BMI of 18.5–24.9 for most UK adults. If you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends a lower range of 18.5–22.9 due to higher health risks at lower BMI levels in these populations.
How does body fat percentage differ from the BMI formula?
BMI only uses weight and height, whilst body fat percentage measures the actual proportion of fat in your body. Two people with the same BMI can have very different body fat percentages depending on muscle mass. Body composition gives a clearer picture of health than BMI alone, but requires specialist measurement.
Can I lose weight without changing my BMI?
Initially, no — if you lose weight, your BMI will decrease proportionally. However, if you lose fat whilst gaining muscle (through exercise), your weight might stay the same but your body fat percentage improves. This is why tracking body fat percentage alongside BMI and waist circumference is valuable for monitoring health progress. ```
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.