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 Can I Work Out My Body Mass Index?
Working out your body mass index (BMI) is straightforward and takes just two measurements: your weight and height. BMI is a useful screening tool that helps assess whether your weight falls within a healthy range according to NHS guidelines. This guide explains the formula, how to calculate it yourself, and what your result means for your health in a UK context.
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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.
The BMI Formula and Step-by-Step Calculation
BMI is calculated using this simple formula: weight (kg) ÷ height (m)². To work it out manually, first convert your height to metres (for example, 173 cm = 1.73 m), then square that number (1.73 × 1.73 = 2.99). Next, divide your weight in kilograms by this squared height figure. For example, if you weigh 75 kg and are 1.73 m tall: 75 ÷ 2.99 = 25.1 BMI.
If you prefer not to calculate manually, use our free BMI calculator above — it handles the maths instantly and shows where you fall on the NHS scale. The beauty of BMI is that it's the same calculation for everyone, though your starting point (your current body weight and lean body mass composition) affects the result.
- Metric formula: weight (kg) ÷ height (m)²
- Convert height to metres first (cm ÷ 100)
- Square your height in metres
- Divide your weight by the squared height
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.
Understanding Your BMI Result and Limitations
Once you have your BMI number, compare it to the NHS BMI categories: below 18.5 is underweight, 18.5–24.9 is healthy weight, 25.0–29.9 is overweight, and 30.0+ is obese. However, BMI has important limitations — it doesn't distinguish between muscle and fat, so athletes or people with high lean body mass may have a high BMI despite low body fat percentage. BMI also doesn't account for where fat is stored; visceral fat around organs poses greater health risk than subcutaneous fat elsewhere.
For a more complete picture of your health, consider your waist circumference alongside BMI, or discuss body composition assessment with your GP. Our free weight converter can help you track your weight in stone, lbs, or kg as you monitor changes over time.
- 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 is a ratio of weight to height and doesn't distinguish between muscle and fat. Body fat percentage measures the actual proportion of your weight that is fat tissue. Someone with high muscle mass may have a high BMI but low body fat percentage. A body fat calculator or DEXA scan gives more detail than BMI alone, but BMI remains a useful screening tool for population-level health assessment.
Do I need to convert my weight to kilograms to calculate BMI?
The standard BMI formula uses kilograms and metres. If your weight is in stone and pounds, convert to kilograms first (1 stone = 6.35 kg, 1 lb = 0.454 kg). Our weight converter tool above makes this instant. Once converted, follow the metric formula: weight (kg) ÷ height (m)².
Is BMI accurate for everyone, including athletes and older adults?
BMI is less accurate for athletes (due to high muscle mass), elderly people (due to age-related muscle loss), and pregnant women. If you're very muscular, have very low muscle mass, or have other health concerns, ask your GP about additional body composition measures alongside BMI for a fuller health picture.
Are NHS BMI categories the same for all ethnic backgrounds?
No. The NHS recommends lower thresholds for adults of South Asian, Chinese, Black African, and Black Caribbean heritage due to higher cardiovascular and diabetes risk at lower BMI levels. For these groups, healthy weight is 18.5–22.9 instead of 18.5–24.9. Always discuss your personal health context with your GP.
What should I do if my BMI is in the overweight or obese range?
If your BMI is above 24.9, discuss this with your GP. They can assess your overall health, check for weight-related conditions, and support sustainable changes through diet, activity, or referral to NHS weight management services. Even a 5–clinically studied weight loss can improve health outcomes. Use our free BMI calculator regularly to track progress.
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.