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 To Calculate Body Mass Index
Body Mass Index (BMI) is a simple measurement that helps assess whether your weight is healthy for your height. Calculating your BMI takes just seconds — divide your weight in kilograms by your height in metres squared — but understanding what your result means is equally important. The NHS uses BMI as a screening tool to identify health risks, though it has limitations. Use our free BMI calculator above to find your result instantly, then read on to understand how to interpret it and how BMI fits into your overall body composition picture.
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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 To Read Your BMI Calculator Results
Once you've entered your height and weight into our BMI calculator above, your result falls into one of five NHS categories. A BMI between 18.5 and 24.9 is considered a healthy weight for most UK adults. Between 25.0 and 29.9 is overweight, 30.0 to 34.9 is obese (Class I), 35.0 to 39.9 is obese (Class II), and 40.0 and above is severely obese (Class III). Below 18.5 is underweight.
Important: if you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds — a healthy range is 18.5 to 22.9, with overweight starting at 23.0. This matters because these groups face higher health risks from excess weight at lower BMI levels.
Remember that individual results vary based on age, muscle mass, and overall fitness. Your GP can provide personalised guidance on what your BMI means for your specific health.
Clinically BackedWeight Loss
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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.
BMI Versus Body Fat Percentage and Other Metrics
BMI is useful but incomplete. It doesn't distinguish between muscle and fat — an athlete might have a high BMI but very low body fat percentage because muscle weighs more than fat. For a fuller picture of your body composition, consider additional metrics: body fat percentage (typically measured by DEXA scan, bioelectrical impedance, or skinfold tests), waist to height ratio (healthy if below 0.5), and waist circumference alone.
Combining BMI with waist circumference gives a better health risk assessment. The NHS recommends checking waist measurement — over 94cm in men or 80cm in women indicates increased cardiovascular risk. Use our free weight converter to cross-reference your measurements across different units, helping you track changes more clearly.
- 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's the BMI formula and how do I calculate it manually?
The formula is: weight (kg) ÷ height (metres)². For example, if you weigh 70kg and are 1.73m tall: 70 ÷ (1.73 × 1.73) = 70 ÷ 2.99 = 23.4 BMI. However, our free calculator above does this instantly — simply enter your weight and height and your result appears immediately.
Does my BMI change if I gain or lose weight?
Yes, BMI changes directly with weight change. Losing 1kg typically lowers your BMI by approximately 0.5–1 point depending on your height. If you're tracking weight loss over time, use our free progress tracker on this site to monitor your BMI changes alongside your weight loss journey.
Is BMI accurate for very tall or very short people?
BMI becomes less accurate at extreme heights. Very tall individuals may have a misleadingly high BMI, whilst very short people may have a misleadingly low BMI. If you're significantly taller or shorter than average (under 152cm or over 190cm), discuss your results with your GP, as they may need additional context.
Can I have a healthy BMI but still have too much body fat?
Yes, this is possible, especially in sedentary individuals. Your BMI might be 24.9 (healthy), but if you have low muscle mass and high lean body mass is missing, your actual body fat percentage could be unhealthy. This is another reason to consider waist measurement and fitness level alongside BMI.
How often should I calculate my BMI?
If you're managing your weight, calculating monthly is reasonable — more frequent changes won't show meaningful shifts. If you're using prescription weight loss medication like Mounjaro or Wegovy, tracking BMI monthly helps monitor progress. Our progress tracker makes this simple and provides trend data over time.
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.