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 Determine Your BMI Body Mass Index
Your body mass index (BMI) is a simple measurement of body weight relative to height, and it's one of the quickest ways to check if your weight falls within a healthy range according to NHS guidelines. Calculating your BMI takes just two pieces of information — your weight and height — and the result gives you an instant snapshot of whether you're in the underweight, healthy weight, overweight, or obese category. This guide explains exactly how to determine your BMI, what the different ranges mean, and how to use this information to understand 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.
How to Calculate Your BMI and Understand the Results
Determining your BMI is straightforward using the BMI formula. The NHS uses the metric formula: divide your weight in kilograms by your height in metres squared. For example, if you weigh 70 kg and are 1.73 m tall, your calculation would be 70 ÷ (1.73 × 1.73) = 23.4 BMI.
Once you have your BMI number, compare it against 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)
For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds — a healthy BMI range is 18.5–22.9, and overweight begins at 23.0. Use our free BMI calculator above to determine your BMI instantly 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.
BMI Limitations and What Else Matters
While BMI is a useful screening tool, it has important limitations. BMI does not distinguish between muscle and fat — athletes or people with high muscle mass may have a high BMI but low body fat. BMI also doesn't account for where fat is stored; waist circumference matters because fat carried around the stomach poses greater health risks than fat stored elsewhere.
For a more complete picture of your health, the NHS recommends also measuring your waist circumference. Men with a waist over 94 cm (37 inches) or women over 80 cm (31.5 inches) have increased health risk, regardless of BMI. Your GP can provide personalised advice about what your BMI means for your individual health, especially if you have underlying health conditions or are very muscular.
- 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 body mass index formula?
The BMI formula is weight (kg) ÷ height (m)². For example, if you weigh 75 kg and are 1.70 m tall: 75 ÷ (1.70 × 1.70) = 25.95 BMI. You can also use our free weight converter to switch between stone and kilograms before calculating.
What is a healthy BMI range according to the NHS?
The NHS defines healthy BMI as 18.5–24.9 for most adults. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the healthy range is lower: 18.5–22.9. Your GP can advise on your personal healthy weight range.
Why doesn't BMI account for muscle mass?
BMI only measures total weight relative to height — it cannot distinguish between muscle tissue and fat tissue. Very muscular athletes may have a "high" BMI but low body fat. This is why BMI should always be considered alongside waist measurement and GP advice.
What should I do if my BMI falls in the overweight category?
If your BMI is 25–29.9, talk to your GP about your weight and health. Small lifestyle changes — like increasing physical activity and reviewing diet — can help. The NHS recommends losing no more than 0.5–1 kg per week for sustainable results.
Is BMI different for children?
Yes. Children's BMI uses age and sex-specific centile charts, not adult categories. If you have concerns about a child's weight, ask your GP for a referral to the NHS Healthy Weight Calculator designed specifically for children.
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.