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 B.M.I Stand For?
B.M.I stands for Body Mass Index — a measurement that uses your weight and height to estimate whether you're at a healthy weight. The NHS uses BMI as a screening tool to assess whether adults fall into healthy, overweight, or obese categories. It's calculated the same way worldwide but the UK applies specific thresholds that align with NHS health guidance. Understanding what BMI means is the first step in assessing your weight health, and our free BMI calculator can instantly show you your personal BMI and what category it falls into.
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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 Is BMI Calculated?
BMI is calculated using a simple mathematical formula:
- Metric formula: weight in kilograms ÷ (height in metres squared) = BMI
- Imperial approximation: (weight in pounds × 703) ÷ (height in inches squared) = BMI
For example, if you weigh 70 kg and are 1.75 metres tall, your BMI would be 70 ÷ (1.75 × 1.75) = 22.9 — which falls in the healthy weight range according to NHS guidance.
The result is always a single number. The NHS then uses this number to place you into a category: underweight (below 18.5), healthy weight (18.5–24.9), overweight (25.0–29.9), obese (30.0+), or severely obese (40.0+). You can calculate your own BMI instantly using our free BMI calculator above, which handles the maths for you and shows where your weight sits on the NHS scale.
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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 Categories and NHS Thresholds
The NHS defines five standard BMI categories for adults. A healthy weight falls between 18.5 and 24.9; overweight is 25.0 to 29.9; obese (Class I) is 30.0 to 34.9; obese (Class II) is 35.0 to 39.9; and severely obese is 40.0 and above. Important: if you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds because these groups face higher health risks at lower BMI levels. For these groups, healthy weight starts at 18.5 and overweight begins at 23.0. Understanding your BMI category helps your GP assess your health risk and whether weight management support might be beneficial. Try our free weight converter to see your weight in different measurements as part of your wider health assessment.
- 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's not perfect?
BMI is a quick, free screening tool that works well for most adults. It helps GPs identify who may be at increased health risk. However, BMI has limitations — it doesn't distinguish between muscle and fat, so athletes may have high BMI despite low body fat. It's one tool among many; your GP may also consider waist measurement, fitness level, and medical history.
Is BMI the same for men and women?
Yes, BMI is calculated the same way for both men and women using the same formula and NHS thresholds. However, research shows men and women may carry fat differently, and some research suggests women may benefit from considering waist circumference alongside BMI for a fuller health picture.
At what age does BMI start to apply?
BMI categories only apply to adults aged 18 and over. Children and teenagers use age and sex-specific centile charts instead — their BMI is compared to UK growth reference data. If you're concerned about a child's weight, consult your GP or use NHS resources designed specifically for children.
Can BMI be high if I have a lot of muscle?
Yes. Muscle weighs more than fat, so very muscular individuals (athletes, bodybuilders) may have a high BMI despite having low body fat. If you're very muscular, discuss your BMI with your GP — they may recommend additional measurements like waist circumference or body composition analysis for a clearer picture.
What should I do if my BMI suggests I'm overweight?
If your BMI falls in the overweight or obese range, start by talking to your GP. They can discuss your individual health, any weight-related conditions, and whether lifestyle changes or weight management support might help. The NHS offers free resources, weight management programmes, and in some cases, prescription support. Our free progress tracker can help you log changes if you decide to 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.