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 Determine Body Mass Index for UK Adults
Body mass index (BMI) is a measurement that helps you understand whether your weight is healthy for your height. It's calculated by dividing your weight in kilograms by your height in metres squared. Our free BMI calculator on this page does this instantly for you. BMI is a useful screening tool recommended by the NHS, though it has limitations — it doesn't distinguish between muscle and fat, so it works best alongside other health measures like body fat percentage and waist circumference.
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 to Interpret Your BMI Results
Once you've calculated your BMI using the calculator above, you can compare it against NHS BMI categories. A healthy BMI for most UK adults is between 18.5 and 24.9. If your BMI falls between 25.0 and 29.9, this is classified as overweight. A BMI of 30.0 or above is classified as obese, with further categories: Class I (30–34.9), Class II (35–39.9), and Class III (40+).
Important: If you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds. Your healthy weight range would be 18.5–22.9, and overweight starts at 23.0. This is because these populations face higher health risks from excess weight at lower BMI levels.
- Use the weight converter on this page to check your weight in stone, pounds, or kilograms
- Remember that BMI doesn't account for muscle mass — athletes may have a high BMI but low body fat
- Combine your BMI result with other health markers for a fuller picture of your health
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.
Body Mass Index vs Body Composition Metrics
While BMI is a quick screening tool, body composition (the ratio of fat to lean body mass) provides more detailed health insight. Body fat percentage directly measures how much of your body weight is fat versus muscle, bone, and other tissue. For women, a healthy body fat percentage is typically 21–24%; for men, 14–17%. You can measure body fat using calipers, bioelectrical impededance, or DEXA scans.
Lean body mass also matters — someone with high muscle content may have a higher BMI but lower health risk than someone at the same BMI with high body fat. A body fat calculator or waist-to-height ratio provides complementary information to BMI. For the most complete picture of your health, use BMI as a starting point, then discuss other metrics like visceral fat (harmful fat around organs) with your GP if you have health concerns.
- 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 correct formula to determine body mass index?
BMI is calculated as weight in kilograms divided by height in metres squared (BMI = kg ÷ m²). For example, someone weighing 70kg and 1.73m tall would have a BMI of 23.4. Our free BMI calculator above does this instantly — just enter your weight and height, and it calculates your exact BMI automatically.
Why do South Asian and Black adults have different BMI thresholds in the UK?
Research shows that South Asian, Chinese, Black African, and Black Caribbean populations develop health complications from excess weight at lower BMI levels. The NHS therefore recommends these groups use a healthy BMI range of 18.5–22.9 (instead of 18.5–24.9) and consider BMI 23+ as overweight. This reflects genuine differences in metabolic risk and helps catch health issues earlier.
How does body fat percentage differ from BMI?
BMI measures weight relative to height but doesn't distinguish between fat and muscle. Body fat percentage directly measures the proportion of your weight that is fat tissue. Two people with identical BMI could have very different body fat percentages — one muscular, one less so. BMI is a quick screening tool; body fat percentage is more precise but requires specialist measurement.
Is a high BMI always unhealthy?
Not necessarily. Athletes and very muscular individuals may have high BMI but low body fat and excellent health. However, BMI remains a useful population-level screening tool. If your BMI is elevated but you're very fit, discuss your individual health with your GP. They can assess your blood pressure, blood sugar, cholesterol, and waist circumference alongside BMI for a complete picture.
Can I use BMI if I'm pregnant?
BMI is less reliable during pregnancy because weight gain is normal and necessary. The NHS advises pregnant women to discuss healthy weight gain with their midwife or GP individually, rather than relying on standard BMI categories. If you're planning pregnancy or currently pregnant, your healthcare team can give personalised guidance.
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.