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 Mass Body Index (BMI) and Why Does It Matter?
Mass body index, or BMI, is a measurement of body weight relative to height — a simple screening tool used by the NHS to assess whether an adult's weight falls within a healthy range. It's calculated by dividing weight in kilograms by height in metres squared. Whilst BMI has limitations, it remains the standard UK health guide for identifying whether someone may be at increased health risk due to excess weight. Understanding your BMI is the first step towards making informed decisions about your health and weight management.
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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 to Calculate Your BMI and What the Numbers Mean
The BMI formula is straightforward: BMI = weight (kg) ÷ height (m)². For example, if you weigh 75kg and are 1.73m tall, your BMI would be 75 ÷ (1.73 × 1.73) = 25.1.
The NHS has set standard BMI categories that apply to most UK adults:
- Underweight: BMI below 18.5
- Healthy weight: BMI 18.5–24.9
- Overweight: BMI 25.0–29.9
- Obese (Class I): BMI 30.0–34.9
- Obese (Class II): BMI 35.0–39.9
- Severely obese (Class III): BMI 40.0+
However, if you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds because these groups face higher cardiovascular and diabetes risk at lower BMI levels. Use our free BMI calculator above to check your personal BMI instantly and see where you fall within these NHS categories.
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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 Your Score Really Tells You
Whilst BMI is a useful screening tool, it has important limitations. BMI does not distinguish between muscle and fat — athletes or very muscular individuals may have a high BMI but low body fat. It also doesn't account for where fat is stored; a high waist circumference carries greater health risk even at the same BMI. BMI is less accurate for older adults (who lose muscle naturally) and during pregnancy.
Your GP can use BMI alongside other health measurements, including waist circumference and blood pressure, to give personalised health advice. If you're concerned about your weight or health, your GP can discuss whether weight management support — including lifestyle changes, dietary adjustments, or in some cases prescription weight management options — might be appropriate for you.
- 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 difference between BMI and body fat percentage?
BMI uses only height and weight, whilst body fat percentage measures the actual proportion of fat in your body. BMI is quicker and cheaper to calculate but doesn't distinguish between muscle and fat. Body fat percentage is more accurate but requires specialist equipment. Both are useful health guides.
Can I convert my weight to stone and check BMI at the same time?
Yes. Use our free weight converter above to switch between kg, stone, and pounds, then use our BMI calculator to check your score instantly. This helps you understand your weight in familiar UK measurements and see where you fall on the NHS health scale.
Does BMI apply differently to men and women?
No, the same BMI categories apply to both men and women. However, research suggests women may carry more health risk at slightly higher BMI levels due to differences in fat distribution. Your GP can discuss what's healthy for you personally.
What should I do if my BMI is in the overweight or obese range?
Start by discussing your health with your GP. They can assess your overall health, family history, and any weight-related conditions. Even a 5–clinically studied weight loss can bring meaningful health benefits. Your GP may suggest lifestyle changes, referral to a weight management programme, or in some cases, prescription weight management treatment.
Are there conditions where BMI is not reliable?
Yes. BMI is less accurate for athletes (high muscle mass), older adults (muscle loss), children (use age-specific charts), pregnant women, and people with certain medical conditions. Your GP can advise whether BMI is a useful measure for your individual situation.
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.