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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Why BMI Isn't Always Accurate
BMI is a useful screening tool, but it has significant limitations that mean it doesn't tell the whole story about your health. The BMI formula measures weight against height but can't distinguish between muscle and fat, doesn't account for where fat is stored, and may not apply equally across different ethnic groups. Understanding these limitations helps you interpret your BMI result more accurately and use it alongside other health measures recommended by the NHS.
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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.
The Main Limitations of BMI
The BMI formula divides your weight in kilograms by your height in metres squared, giving a single number that falls into categories defined by the NHS: underweight (below 18.5), healthy weight (18.5–24.9), overweight (25–29.9), and obese (30 and above). However, this simple calculation ignores several important factors.
- Muscle vs fat: Muscle weighs more than fat. Athletes and people with high muscle mass often have a high BMI despite low body fat, leading to misclassification as overweight or obese when they're actually fit.
- Fat distribution: Where your body stores fat matters for health risk. Visceral fat (around organs) poses greater health risks than subcutaneous fat (under skin), but BMI doesn't distinguish between them.
- Age and bone density: Older adults lose muscle and bone density naturally, which affects BMI interpretation. BMI may underestimate health risk in elderly people.
- Ethnicity adjustments: The NHS recommends lower BMI thresholds for South Asian, Chinese, Black African, and Black Caribbean populations because these groups face higher diabetes and cardiovascular risk at lower BMI levels.
Use our free BMI calculator to check your personal BMI, but remember these limitations when interpreting your result.
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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.
Better Health Measures Beyond BMI
The NHS recommends using BMI alongside other health measures for a fuller picture. Waist circumference is particularly important: men over 94 cm and women over 80 cm face increased health risk, with higher risk at 102 cm (men) and 88 cm (women). Waist-to-height ratio (waist circumference ÷ height) is considered a better predictor of cardiovascular risk than BMI alone — a healthy ratio is below 0.5 for all adults.
Body composition analysis, including body fat percentage measured by DEXA, bioelectrical impedance, or skin calipers, gives much more detail about muscle and fat proportions. Your GP can assess your overall health through blood pressure, cholesterol levels, blood sugar, and fitness — none of which BMI captures. Rather than relying on BMI alone, use our weight converter and BMI calculator together as screening tools, then discuss your individual health picture with your GP.
- 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.
Can I have a high BMI but still be healthy?
Yes. Athletes and muscular individuals often have high BMI from muscle mass rather than excess fat. Conversely, someone with a "healthy" BMI might have poor cardiovascular fitness or unhealthy fat distribution. BMI is a screening tool only — it doesn't measure fitness, strength, or actual body composition.
Does ethnicity affect BMI accuracy?
Yes. The NHS recommends lower BMI thresholds for South Asian, Chinese, Black African, and Black Caribbean adults because these populations face higher type 2 diabetes and heart disease risk at lower BMI levels. Standard BMI categories don't account for these differences, potentially delaying diagnosis of health risk.
Why doesn't BMI account for muscle?
The BMI formula only uses total weight and height — it can't distinguish between muscle and fat tissue. Since muscle is denser than fat, someone who weighs 80kg might have very different body composition depending on whether that weight is from muscle or fat.
What's a better alternative to BMI?
Waist circumference and waist-to-height ratio are considered stronger predictors of health risk. Combining BMI with these measures, plus fitness assessments and blood work, gives your GP a much clearer picture of your actual health than BMI alone.
Should I ignore my BMI entirely?
No — BMI remains a useful first screening tool, especially at population level. Just don't rely on it alone. Use your BMI result as a starting point for a conversation with your GP about your overall health, fitness, and individual risk factors.
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.