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 Index? A Clear UK Definition
Body index is a measure of body fat based on your height and weight. In the UK, the most commonly used body index is BMI (Body Mass Index), which NHS guidance uses to assess whether an adult's weight falls within a healthy range. Understanding your body index is an important part of UK health information and weight management, though it's just one of several tools your GP may consider when evaluating your overall health. This guide explains what body index means, how it's calculated, and what the NHS recommends.
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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.
How Body Index Is Calculated in the UK
Body Mass Index (BMI) is the standard body index measure used by the NHS. It's calculated using a simple formula: weight in kilograms divided by height in metres squared. For example, if you weigh 70kg and are 1.75m tall, your BMI would be 70 ÷ (1.75 × 1.75) = 22.9.
The NHS groups BMI into five categories: underweight (below 18.5), healthy weight (18.5–24.9), overweight (25–29.9), obese (30–34.9), and severely obese (40+). For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds because these groups face higher health risks at lower BMI levels.
Use our free BMI calculator above to check your personal body index instantly and see where you fall within the NHS healthy weight range.
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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.
Body Index Limitations and What Else Matters
While body index is useful as an initial screening tool, the NHS recognises it has important limitations. BMI does not distinguish between muscle and fat—athletes and very muscular individuals may have a high body index but low body fat. BMI also does not account for where fat is stored, which matters for health; fat around the waist poses greater cardiovascular risk than fat elsewhere.
Your GP may also consider your waist circumference, fitness level, family health history, and blood pressure when assessing your health. For a full nhs health advice picture, discuss your body index results with your GP, who can give personalised guidance based on your individual circumstances and any weight-related health conditions.
- 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 body index the same as BMI?
In UK health guidance, body index and BMI (Body Mass Index) are used interchangeably. BMI is the standard body index measure used by the NHS. However, "body index" is a broader term that could theoretically include other measurements like body fat percentage or waist-to-height ratio.
What body index should I aim for?
The NHS recommends a healthy body index (BMI) of 18.5 to 24.9 for most UK adults. For adults of South Asian, Chinese, Black African, or Caribbean heritage, the healthy range is 18.5 to 22.9 due to higher health risk at lower BMI levels. Your GP can advise on a target appropriate for you.
Can I convert my body index between kg and stone?
Yes. Use our free weight converter above to switch between kilograms and stone, then recalculate your body index. Alternatively, our BMI calculator works with both metric and imperial measurements, so you can input your weight in stone and get your body index instantly.
Does body index tell me if I'm overweight?
Body index gives useful initial information, but it's not the complete picture. A high body index may indicate excess weight, but it doesn't distinguish muscle from fat. Your GP considers body index alongside other factors like waist measurement, fitness, and health conditions when determining whether your weight poses health risks.
Where can I get UK health guidance on body index?
The NHS website offers free health information on healthy weight and BMI. Your GP can provide personalised nhs guidance based on your body index and medical history. MyWeightCheck also offers free health information tools and calculators to help you understand your weight management health context.
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.