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 Body Mass Indicator and How Does It Work in the UK?
Body mass indicator (BMI) is a measure of body weight in relation to height, used by the NHS and healthcare professionals across the UK to assess whether adults are at a healthy weight. BMI is calculated using a simple formula: your weight in kilograms divided by your height in metres squared. It provides a quick screening tool to identify whether someone may be at increased health risk due to excess weight, though it has important limitations you should understand.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 BMI Is Calculated and NHS Weight Categories
The BMI formula is: weight (kg) ÷ height (m)². For example, someone weighing 70kg with a height of 1.73m (5ft 8in) would have a BMI of approximately 23.4, falling in the healthy weight range.
The NHS uses these standard BMI 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 people of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower BMI thresholds because these groups face higher health risks at lower weight levels. For these populations, a healthy BMI is 18.5–22.9, and overweight begins at 23.0. Use our free BMI calculator above to check your personal BMI instantly and see where you fall on the NHS scale.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 Complementary Health Measures
While useful as a screening tool, BMI has significant limitations. It does not distinguish between muscle and fat—athletes may have a high BMI but low body fat percentage, which is actually a more accurate measure of health. BMI also ignores where fat is stored; visceral fat around organs is more harmful than subcutaneous fat under the skin.
For a more complete picture of your body composition, consider measuring your waist circumference and lean body mass. The NHS recommends: men over 94cm waist = increased risk, over 102cm = high risk; women over 80cm = increased risk, over 88cm = high risk. Your GP can discuss your individual health using BMI alongside these measures to give personalised 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 body mass indicator and body mass index?
There is no difference. "Body mass indicator" and "body mass index" (BMI) are the same thing—they're just different names for the same measurement. In the UK, you'll usually hear it called BMI. The term "indicator" emphasises that BMI is a screening tool, not a definitive health diagnosis.
Is BMI accurate for everyone in the UK?
BMI is useful for most adults but has limitations. It doesn't account for muscle mass, age, or ethnic background. The NHS recommends lower BMI thresholds for South Asian, Chinese, Black African, and Black Caribbean populations. If you're very muscular, elderly, or pregnant, BMI alone may not give an accurate picture—your GP can assess your health more comprehensively.
What is considered a healthy BMI in the UK?
According to NHS guidelines, a healthy BMI for most UK adults is 18.5–24.9. For people of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends 18.5–22.9. You can check your personal healthy weight range using our BMI calculator above to see the exact range for your height.
How does body fat percentage relate to BMI?
Body fat percentage measures the actual proportion of fat in your body, while BMI estimates it indirectly from weight and height. Someone with a high BMI might have high muscle mass rather than high fat. Body fat percentage is more specific but harder to measure accurately. Both together give a more complete picture of body composition than BMI alone.
Can I lose weight and improve my BMI safely?
Yes. The NHS recommends losing weight at a rate of 0.5–1kg per week for sustainable results. This requires a calorie deficit of around 500 kcal per day alongside increased physical activity. Track your progress with our free progress tracker, and consult your GP if you want personalised guidance on safe weight loss for your circumstances.
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.