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 Index for Obesity?
Body mass index (BMI) is a measurement that classifies weight in relation to height, and is used by the NHS to identify obesity. A BMI of 30.0 or above falls into the obesity category according to UK health guidelines. However, BMI is just one screening tool — it doesn't distinguish between muscle and fat, and doesn't account for where fat is stored on your body. Understanding your BMI and what it means for your health is an important first step, but it's best combined with other measures like waist circumference and body fat percentage for a fuller picture of your health status.
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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.
BMI Categories and Obesity Classification
The NHS classifies BMI into five categories, with obesity starting at BMI 30.0:
- 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 and above
For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds due to higher cardiovascular risk at the same BMI. For these groups, obesity starts at BMI 27.5 or above. You can calculate your personal BMI instantly using our free BMI calculator above to see which category applies to you.
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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 Body Composition Context
While BMI is a useful screening tool, it has significant limitations. BMI doesn't distinguish between lean body mass (muscle) and fat tissue — an athlete with high muscle mass may have a high BMI but low body fat percentage. It also ignores where fat is stored; visceral fat (around organs) is more harmful than subcutaneous fat (under skin). For a more complete assessment of your health, consider measuring your waist circumference (above 94cm for men or 80cm for women indicates increased risk) alongside BMI. Our weight converter and progress tracker can help you monitor changes over time, while discussing your overall health picture with your GP ensures you're getting personalised guidance based on your individual circumstances, genetics, and any weight-related health conditions.
- 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 BMI is considered obese in the UK?
According to NHS guidelines, a BMI of 30.0 or above is classified as obese. This is divided into Class I (30.0–34.9), Class II (35.0–39.9), and severely obese or Class III (40.0+). For people of South Asian, Chinese, Black African, or Black Caribbean heritage, obesity classification starts at BMI 27.5 due to higher health risks at lower BMI levels.
How is BMI calculated and why does it matter for obesity screening?
BMI is calculated by dividing your weight in kilograms by your height in metres squared (kg÷m²). It matters because it's a quick screening tool used by the NHS to identify whether someone's weight may pose health risks. However, BMI alone doesn't tell you about body fat percentage or lean muscle mass, so it should be combined with other assessments for a complete health picture.
Why do people of different ethnicities have different BMI thresholds?
Research shows that people of South Asian, Chinese, Black African, and Black Caribbean descent have higher cardiovascular and type 2 diabetes risk at lower BMI levels compared to white European populations. This is why the NHS recommends lower BMI thresholds (starting at 23 for overweight, 27.5 for obesity) for these groups, reflecting their individual health profiles rather than a one-size-fits-all approach.
Does a high BMI always mean someone has high body fat?
No. BMI doesn't distinguish between muscle and fat, so athletes, bodybuilders, or very muscular individuals may have a high BMI but low body fat percentage. For a more accurate assessment of your body composition, consider measuring waist circumference, checking your body fat percentage, and discussing your health with your GP — especially if you have significant muscle mass.
What should I do if my BMI falls in the obesity range?
If your BMI is 30 or above, speaking with your GP is a good first step. They can assess your overall health, discuss any weight-related health conditions, and explore appropriate options which may include lifestyle support, diet and exercise guidance, or — if suitable — prescription weight management treatments. Use our free BMI calculator to track your current BMI, and our progress tracker to monitor changes over time as you make adjustments.
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.