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 Do You Measure Your BMI Index?
Measuring your BMI index is straightforward — it's a simple calculation based on your weight and height. BMI (body mass index) is a useful screening tool that helps assess whether your weight is healthy for your height. The NHS uses BMI as a primary indicator of healthy weight, though it works best alongside other health measurements. Understanding how to calculate your body mass index and where your result falls within NHS categories can help you make informed decisions about your health and when to speak with your GP.
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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.
NHS BMI Formula and Categories
The BMI formula is straightforward: divide your weight in kilograms by your height in metres squared (kg ÷ m²). For example, if you weigh 75kg and are 1.73m tall, your BMI would be 75 ÷ (1.73 × 1.73) = 25.1.
Once calculated, the NHS divides BMI into clear 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)
You can use our free BMI calculator above to find your result instantly without a UK pharmacy calculation. If you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds — your healthy BMI range is 18.5–22.9 rather than 18.5–24.9.
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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.
NHS Weight Management and When to Seek Help
The NHS recommends a healthy bmi between 18.5 and 24.9 for most adults, though this varies by ethnicity. If your BMI falls outside this range, your GP can offer support through NHS weight management services. NICE guidance (National Institute for Health and Care Excellence) recommends that GPs identify overweight patients and offer evidence-based interventions, which may include structured weight loss programmes, physical activity support, or referral to specialist tier 3 weight management services.
If you're concerned about your weight or health, your GP is the best starting point. They can assess whether your BMI relates to genuine health risks, consider your individual circumstances, and discuss weight management options tailored to you. Use the weight converter on this page if you need to convert your weight between kg, stone, and pounds for NHS records.
- 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.
Is BMI an accurate way to measure health?
BMI is a useful screening tool but has limitations. It doesn't distinguish between muscle and fat — athletes may have a high BMI but low body fat. It also doesn't account for where fat is stored (waist circumference matters too) or factors like age and bone density. The NHS uses BMI alongside other measurements for a complete health picture.
What's the difference between overweight and obese according to the NHS?
According to NHS categories, overweight is a BMI of 25.0–29.9, whilst obese is 30.0 or above. The NHS further divides obesity into three classes based on health risk. These are official health thresholds used to identify when weight may affect health and when intervention might be helpful.
Why does the NHS use lower BMI thresholds for some ethnic groups?
Research shows that people of South Asian, Chinese, Black African, and Black Caribbean heritage have higher risks of type 2 diabetes and cardiovascular disease at the same BMI as other populations. The NHS therefore recommends lower BMI thresholds (18.5–22.9 as healthy) for these groups to better reflect individual health risk.
What should I do if my BMI falls in the overweight range?
If your BMI is 25–29.9, speak with your GP. They can assess your individual health, check for weight-related conditions like high blood pressure, and discuss whether lifestyle changes or weight management support would benefit you. The NHS offers free weight management support through various programmes.
Can I use imperial measurements (stone and pounds) to calculate BMI?
BMI calculation requires metric units (kg and metres). If you measure in stone and pounds, convert to kg first, or use our free weight converter above. Alternatively, use our free BMI calculator which accepts stone and pounds directly and shows your result instantly.
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.