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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What Is Body Index and Why Does It Matter?
Body index, more commonly known as BMI (Body Mass Index), is a simple measurement that helps assess whether your weight falls within a healthy range for your height. It's one of the main tools used by the NHS to screen for weight-related health risks in UK adults. While BMI has limitations, it remains a useful starting point for understanding your health status and can guide conversations with your GP about weight management. Our free BMI calculator above can help you find your personal body index instantly, and we've created this UK health guide to explain what your result means.
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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 Body Index Is Calculated and What It Means
Body index (BMI) is calculated using a simple formula: your weight in kilograms divided by your height in metres squared. For example, if you weigh 70kg and are 1.75m tall, your BMI would be approximately 22.9, which falls in the healthy weight range according to NHS guidance.
The NHS categorises body index as follows:
- Below 18.5 = Underweight
- 18.5 to 24.9 = Healthy weight
- 25.0 to 29.9 = Overweight
- 30.0 to 34.9 = Obese (Class I)
- 35.0 to 39.9 = Obese (Class II)
- 40.0 and above = Severely obese (Class III)
For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds due to higher health risks at the same body index levels. If you want to convert your weight between different units, use our free weight converter to switch between stone, pounds, and kilograms.
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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 Limitations of Body Index
While body index is useful, it has real limitations you should understand. BMI doesn't distinguish between muscle and fat—athletes may have a high body index but low body fat. It also doesn't account for where fat is stored on your body (waist circumference matters too), and it's less accurate for older adults who have lost muscle mass. Despite these limitations, body index remains a practical nhs health advice tool for screening and is often the first step when discussing weight management with your GP. Always combine your body index result with other health measures and professional guidance for the most complete picture of your health.
- 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 difference between body index and body fat percentage?
Body index (BMI) is a simple height-to-weight ratio, whilst body fat percentage measures the actual amount of fat in your body. BMI doesn't distinguish between muscle and fat, whereas body fat percentage does. An athlete might have a high BMI but low body fat. Both are useful measurements, but body fat gives a more detailed picture of body composition.
Can I use body index if I'm pregnant?
BMI is less reliable during pregnancy because pregnancy weight gain is natural and necessary for fetal development. Your GP or midwife will use different standards to assess your health during pregnancy rather than standard body index categories. Always discuss weight management with your healthcare provider if you're pregnant or planning pregnancy.
What should I do if my body index is outside the healthy range?
If your body index falls outside the healthy range, start by speaking with your GP, who can assess your individual health status and recommend next steps. This might include lifestyle changes like increased physical activity and dietary adjustments. In some cases, your GP may refer you to NHS weight management services or discuss prescription weight loss options if appropriate.
Is body index accurate for very tall or very short people?
BMI is generally less accurate at height extremes. Very tall individuals may have a higher body index that doesn't reflect their actual health, whilst very short people may have lower scores. If you're unusually tall or short, discuss your body index results with your GP rather than relying on the number alone.
How often should I check my body index?
Checking your body index monthly or every 3 months is reasonable if you're actively working on weight management. More frequent checks (weekly) aren't necessary and may be discouraging. Use our free progress tracker to monitor your weight loss journey over time alongside regular body index checks.
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.