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 Does BMI Measure?
BMI (Body Mass Index) measures the relationship between your weight and height to estimate whether you're at a healthy weight. It's a screening tool used by the NHS and GPs across the UK to identify potential health risks associated with excess weight. BMI doesn't directly measure body fat or health, but it's a quick way to see if your weight falls into a healthy range. Understanding what BMI measures helps you interpret health conversations with your GP and track your wellness journey.
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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 BMI is Calculated and What the Ranges Mean
BMI is calculated using a simple formula: weight (kg) divided by height (m) squared. For example, if you weigh 70kg and are 1.75m tall, your BMI would be approximately 23, which falls in the healthy range according to NHS guidelines.
The NHS divides BMI into these categories for UK adults:
- Below 18.5: Underweight
- 18.5–24.9: Healthy weight
- 25.0–29.9: Overweight
- 30.0–34.9: Obese (Class I)
- 35.0–39.9: Obese (Class II)
- 40.0+: Severely obese (Class III)
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 the same BMI levels. A healthy BMI for these groups starts at 18.5 and extends to 22.9.
Use our free BMI calculator above to find your personal BMI instantly and see which category you fall into.
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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 Limitations and What Else Matters for Your Health
Although BMI is useful, it has important limitations. BMI doesn't distinguish between muscle and fat — an athlete with high muscle mass may have a high BMI but be perfectly healthy. It also doesn't account for where fat is stored (belly fat carries more health risk than fat elsewhere), your age, or your overall fitness level.
For a fuller picture of your health, your GP may also consider your waist circumference. Men should aim for under 94cm (37 inches), and women under 80cm (31.5 inches). Combining BMI with waist measurement gives a more accurate assessment of health risk than BMI alone. Our free weight converter and BMI calculator help you track both metrics together.
- 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.
Why does the NHS use BMI if it has limitations?
BMI is a quick, simple, and cost-effective screening tool that helps GPs identify people who may be at higher risk of health conditions like type 2 diabetes, heart disease, and high blood pressure. It's not meant to diagnose health problems — it's a starting point for conversation. Your GP uses BMI alongside other health information to assess your individual risk.
Is BMI different for men and women?
No, the BMI formula and categories are the same for both men and women in UK health guidelines. However, women typically have naturally higher body fat percentages than men at the same BMI level, so some health professionals also consider body composition when assessing individual health risks.
What if my BMI is healthy but my waist is large?
If your BMI falls in the healthy range but your waist circumference is above NHS thresholds (over 94cm for men, over 80cm for women), you may still have increased health risks. This pattern suggests fat is concentrated around the abdomen, which carries greater health risk. Discuss this with your GP, who can assess your full health picture.
Can BMI be used for children?
No, adult BMI categories don't apply to children. Children's weight is assessed using age and sex-specific growth charts (UK90 charts in the NHS). If you're concerned about your child's weight, your GP or health visitor can provide appropriate guidance using correct paediatric references.
How often should I check my BMI?
There's no set frequency. If you're monitoring weight changes, checking your BMI monthly or quarterly is reasonable. If you're stable, checking once yearly during a health review is sufficient. Use our free BMI calculator and progress tracker to log changes over time and discuss trends with your GP.
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.