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 Does a BMI Calculator Work?
A BMI calculator is a simple tool that measures your body mass index — a screening tool for body weight relative to height. Our free BMI calculator on this page does the maths instantly, but understanding how it works helps you interpret your result. BMI uses a standard formula applied by the NHS and healthcare professionals worldwide. Whether you're checking your health status or tracking progress, knowing your BMI range is a useful first step in understanding your weight and health.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 the BMI Formula and Ranges
The BMI formula is straightforward: weight in kilograms divided by height in metres squared (kg ÷ m²). For example, if you weigh 70 kg and are 1.73 m tall, your BMI is 70 ÷ (1.73 × 1.73) = 23.4, which falls in the healthy weight range.
The NHS provides clear 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)
For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the healthy BMI range is lower (18.5–22.9), as these groups face higher health risks at standard BMI thresholds. Use our free BMI calculator above to find your personal BMI instantly and see which category you fall into.
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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.
Important BMI Limitations and What Else Matters
BMI limitations are important to understand. BMI does not distinguish between muscle and fat — athletes or very muscular individuals may have a high BMI but low body fat. BMI also doesn't account for where fat is stored; waist circumference is equally important for health risk. The NHS recommends measuring waist circumference alongside BMI: over 94 cm (37 inches) for men or 80 cm (31.5 inches) for women indicates increased health risk.
Your GP can provide personalised interpretation of your BMI in the context of your overall health, family history, and any existing health conditions. BMI is a useful screening tool, but it's one piece of the puzzle, not the whole picture.
- 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 a healthy BMI according to the NHS?
According to NHS guidelines, a healthy BMI is between 18.5 and 24.9 for most adults. For people of South Asian, Chinese, Black African, or Black Caribbean heritage, the healthy range is slightly lower at 18.5 to 22.9. Calculate your personal BMI using our free calculator above to see where you fall.
How do I calculate BMI manually?
The BMI formula is: weight (kg) ÷ height (m)². For example, if you weigh 75 kg and are 1.75 m tall, your BMI is 75 ÷ (1.75 × 1.75) = 24.5. Our BMI calculator does this instantly, but understanding the formula helps you verify your result.
What BMI is considered overweight in the UK?
The NHS defines overweight as a BMI of 25.0 to 29.9. An overweight BMI threshold of 25 means that above this point, you have increased risk of weight-related health conditions. Your GP can advise on whether lifestyle changes or medical support would be helpful for your situation.
Why does BMI vary by ethnicity?
Research shows that people of South Asian, Chinese, Black African, and Black Caribbean backgrounds face higher risk of type 2 diabetes and cardiovascular disease at lower BMI levels compared to white European populations. The NHS recommends using lower thresholds (18.5–22.9 for healthy weight) for these groups to ensure earlier intervention when appropriate.
What are the main BMI limitations?
BMI does not distinguish muscle from fat, so muscular individuals may have a high BMI despite low body fat. It also ignores where fat is stored — waist circumference is equally important. BMI is less accurate for elderly people, children, and pregnant women. Always combine BMI with waist measurement and GP advice for full health context.
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.