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
What Is Body Mass Index (BMI) and How Is It Calculated?
Body mass index (BMI) is a measurement that uses your height and weight to estimate whether you're in a healthy weight range. It's one of the most widely used screening tools by the NHS and healthcare professionals in the UK to assess health risk related to excess weight. While BMI isn't perfect — it doesn't distinguish between muscle and fat — it provides a useful baseline for understanding your weight and overall health. Use our free BMI calculator above to find your personal BMI instantly and discover where you fall on the NHS health categories.
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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.
NHS BMI Categories and Healthy Weight Ranges
The NHS uses five standard BMI categories to classify adult weight. Your BMI formula is straightforward: weight (kg) divided by height (m) squared. For example, an adult who is 1.73m tall and weighs 70kg would have a BMI of approximately 23.4, which falls in the healthy BMI range.
- Underweight: BMI below 18.5
- Healthy weight: BMI 18.5 to 24.9
- Overweight: BMI 25.0 to 29.9
- Obese (Class I): BMI 30.0 to 34.9
- Severely obese (Class III): BMI 40.0 and above
However, people of South Asian, Chinese, Black African, and Black Caribbean backgrounds face higher health risks at lower BMI levels. For these groups, the overweight BMI threshold starts at 23.0 rather than 25.0. Your GP can explain which categories apply to you personally.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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
BMI limitations are important to understand. BMI does not distinguish between muscle and fat — athletes or people with significant muscle mass may have a high BMI despite low body fat. Older adults may have lower muscle mass, affecting their BMI accuracy. Additionally, BMI categories don't account for where fat is stored; waist circumference is equally important. The NHS recommends measuring your waist: over 94cm (37 inches) for men or 80cm (31.5 inches) for women indicates increased health risk. Combine your BMI result with the weight converter tool on this page to understand your weight in stone and kg, then discuss your personal health with your GP for a complete picture of your wellbeing.
- 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.
How do I calculate my BMI?
Divide your weight in kilograms by your height in metres squared. For example: 70kg ÷ (1.73 × 1.73) = BMI of 23.4. Our free BMI calculator above does this instantly for you — just enter your height and weight in your preferred units (kg, stone, or pounds) and it will calculate your BMI and show which NHS category you fall into.
What is a healthy BMI range in the UK?
According to NHS guidelines, a healthy BMI for most UK adults is 18.5 to 24.9. However, for people of South Asian, Chinese, Black African, or Black Caribbean heritage, the healthy range is 18.5 to 22.9 due to higher health risks at lower BMI levels. Your GP can advise which range applies to you.
Why doesn't BMI account for muscle?
BMI uses only height and weight, so it can't distinguish between muscle and fat. Muscle is denser than fat, meaning muscular people may have a higher BMI despite being very healthy. This is why BMI works best as a screening tool alongside other measurements like waist circumference and personal health assessments from your GP.
What should I do if my BMI is in the overweight range?
If your BMI falls in the overweight or obese category, discuss it with your GP. They can assess your individual health, check your waist circumference, and discuss whether lifestyle changes, a structured weight management programme, or other support would be beneficial for you. The NHS recommends losing weight gradually at 0.5 to 1kg per week.
Is BMI different for children?
Yes — children's BMI is calculated the same way but interpreted using age and sex-specific growth charts (UK90 charts), not adult categories. If you're concerned about a child's weight, speak to your GP or health visitor. The NHS Healthy Weight Calculator for children is available on the NHS website and considers their growth pattern.
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.