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 an Index Formula and How Does It Apply to UK Health?
An index formula is a mathematical calculation used to assess health risk and body composition in the UK. The most common is the Body Mass Index (BMI), which uses your weight and height to screen for health conditions. The NHS uses several index formulas to help GPs and patients understand whether weight falls within a healthy range. These standardised measurements help identify whether someone may benefit from lifestyle changes or further health support. Understanding these formulas empowers you to take control of your health journey.
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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.
Common Index Formulas Used in UK Health Guidance
The Body Mass Index (BMI) is the most widely used index formula in UK health assessment. It is calculated as weight in kilograms divided by height in metres squared (kg ÷ m²). The NHS uses BMI categories to screen for health risks: underweight (below 18.5), healthy weight (18.5–24.9), overweight (25–29.9), obese (30–34.9), and severely obese (40+).
For people of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower BMI thresholds due to higher cardiovascular risk at lower weights.
The waist-to-height ratio is another important index formula, calculated as waist circumference divided by height (both in the same units). A healthy ratio is below 0.5 for all adults, regardless of gender. This measure is often more predictive of heart disease risk than BMI alone because it accounts for where fat is stored — a significant factor the BMI formula cannot measure.
Use our free BMI calculator above to check your personal BMI instantly and understand your NHS category.
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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.
UK Health Resources and When to Seek Guidance
The NHS recommends discussing your health metrics with your GP if you fall outside the healthy weight range or have concerns. Your GP can assess your individual situation, consider your family history, activity level, and any existing health conditions — factors that index formulas alone cannot capture.
Many NHS weight management programmes use these index formulas as screening tools at tier 1 and tier 2 services, helping identify who may benefit from specialist support. If you want to track changes in your weight and health metrics over time, our free progress tracker allows you to log your weight regularly and monitor trends. Remember, no single formula tells the complete health story — these are useful screening tools alongside professional medical advice.
- 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 BMI and other index formulas?
BMI is a simple ratio of weight to height used for rapid health screening. Waist-to-height ratio accounts for fat distribution, which BMI cannot measure. Body fat percentage directly measures fat tissue. NHS guidance recommends using multiple measures together for a fuller health picture, as no single formula captures all health risk factors.
Why does the NHS recommend different BMI thresholds for some ethnic groups?
Research shows that people of South Asian, Chinese, Black African, and Black Caribbean heritage have higher risk of type 2 diabetes and heart disease at lower BMI levels. The NHS therefore recommends overweight status begins at BMI 23 (not 25) and obese at 27.5 (not 30) for these groups to catch health risks earlier.
Is BMI a reliable health indicator on its own?
BMI is a useful screening tool but has limitations. It does not distinguish between muscle and fat, so athletes may have high BMI but low body fat. It also ignores where fat is stored and does not account for age, bone density, or medical conditions. Always discuss your BMI with your GP for personalised interpretation.
How often should I recalculate my index formula measurements?
The NHS suggests checking your weight and BMI monthly if managing weight actively, or annually for general health screening. Waist circumference can be measured every 3–6 months. Consistency in measurement method (same time of day, same scales, same tape measure) ensures reliable tracking over time.
Where can I find official NHS guidance on healthy weight?
The NHS website has comprehensive healthy weight guidance, including interactive tools and weight management resources. Your GP surgery can also provide personalised advice based on your health history. MyWeightCheck provides free BMI and weight converters aligned with NHS categories to help you understand your metrics in 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.