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 Is BMI Measured?
BMI (body mass index) is a simple measurement calculated from your weight and height using a standard formula. It's used in the UK by the NHS and your GP to screen for whether you're at a healthy weight and assess health risk. BMI is quick to calculate and useful as a starting point, but it has important limitations — it doesn't distinguish between muscle and fat, for example. Understanding how BMI is measured and what the ranges mean can help you make informed decisions about your health.
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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 BMI Formula and UK Ranges
BMI is calculated using this simple BMI formula: divide your weight in kilograms by your height in metres squared. Mathematically: BMI = weight (kg) ÷ height (m)². For example, if you weigh 70kg and are 1.75m tall, your BMI would be 70 ÷ (1.75 × 1.75) = 22.9.
Once calculated, your BMI falls into one of the NHS BMI categories:
- Below 18.5 = underweight
- 18.5–24.9 = healthy weight (this is the target healthy BMI range)
- 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 people of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS BMI thresholds are lower, starting at 23.0 for the overweight category, because these groups face higher health risks at lower BMI levels. Use our free BMI calculator above to find your exact BMI instantly.
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.
Important BMI Limitations and What Else Matters
The key BMI limitations you should know: BMI cannot tell the difference between muscle and fat, so athletes or very muscular people may have a high BMI but low body fat. BMI also doesn't account for where fat is stored — carrying excess fat around the stomach increases health risk more than fat stored elsewhere. It's less accurate for elderly people who've lost muscle with age, and less reliable during pregnancy.
Because of these limitations, your GP may also measure your waist circumference. For men, a waist above 94cm is increased risk; above 102cm is high risk. For women, above 80cm is increased risk; above 88cm is high risk. Combined with BMI, waist measurement gives a clearer picture of your health. Our free weight converter can help you track changes across kg, stone, and lbs.
- 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 exact BMI formula?
BMI = weight (kg) divided by height (metres) squared. For example, someone weighing 80kg and 1.80m tall would have a BMI of 80 ÷ (1.80 × 1.80) = 24.7, which falls in the healthy weight range according to NHS guidelines.
What is a healthy BMI in the UK?
According to the NHS, a healthy BMI is between 18.5 and 24.9 for most UK adults. However, for people of South Asian, Chinese, Black African, or Black Caribbean heritage, the healthy range is 18.5 to 22.9 because these groups face higher health risks at higher BMI levels.
Why does BMI have limitations?
BMI cannot distinguish between muscle and fat — so a very muscular person or athlete may have a high BMI but low body fat percentage. BMI also ignores where fat is stored and is less accurate for elderly people, pregnant women, and children.
What should I use instead of BMI?
Combine BMI with waist circumference measurement for a fuller picture. Men's waist over 102cm or women's over 88cm indicates increased health risk, even at a normal BMI. Your GP can assess your overall health using multiple measures.
Can I use the BMI calculator on this page?
Yes — use our free BMI calculator above to find your personal BMI instantly. Simply enter your height and weight in either metric or imperial measurements, and the calculator will show your BMI category and healthy weight range according to NHS standards.
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.