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 Take A BMI — Simple Steps & Formula
Taking a BMI (body mass index) is straightforward — it's a simple calculation using your weight and height to estimate whether you're at a healthy weight. BMI is one of the most widely used health screening tools in the UK and is recommended by the NHS for assessing weight-related health risks. You don't need any special equipment; just your weight in kilograms and height in metres. In this guide, we'll walk you through the exact steps, explain the NHS BMI formula, and show you how to interpret your results using UK health thresholds.
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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 & How To Calculate It
The BMI formula is simple and the same for all UK adults: weight (kg) ÷ height (m)². Here's how to calculate step-by-step:
- Step 1: Weigh yourself in kilograms. If you know your weight in stone and pounds, convert it first using our free weight converter.
- Step 2: Measure your height in metres (or convert from feet and inches — 5ft 8in = 1.73m).
- Step 3: Square your height (multiply it by itself). Example: 1.73 × 1.73 = 2.99.
- Step 4: Divide your weight by the squared height. Example: 70kg ÷ 2.99 = 23.4 BMI.
The result is your BMI number. Compare this against the NHS BMI categories to understand where you fall. Use our free BMI calculator above to get your exact result instantly — no a UK pharmacy maths needed.
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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 & What Your Result Means
Once you've taken your BMI, compare your number to the official NHS BMI ranges to find your category:
- Below 18.5: Underweight
- 18.5–24.9: Healthy weight (the target range)
- 25.0–29.9: Overweight
- 30.0–39.9: Obese
- 40.0+: Severely obese
If you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds — the overweight threshold starts at BMI 23.0 instead of 25.0, because these groups have higher cardiovascular risk at lower BMI levels. Your GP can discuss what your specific BMI means for your personal health and whether any action is needed.
- 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 BMI formula exactly?
BMI = weight in kilograms divided by height in metres squared. For example, if you weigh 75kg and are 1.75m tall: 75 ÷ (1.75 × 1.75) = 75 ÷ 3.06 = 24.5 BMI. This places you in the healthy weight range according to NHS guidelines.
Can I calculate BMI if I only know my weight in stone?
Yes. First convert your weight to kilograms (1 stone = 6.35kg), then use the BMI formula. Alternatively, use our free weight converter on this page to switch between stone, pounds, and kilograms instantly, then check your BMI.
Is BMI accurate for everyone?
BMI is a useful screening tool but has limitations. It doesn't distinguish between muscle and fat — athletes may have a high BMI but low body fat. It also doesn't account for where fat is stored. Your waist measurement and overall health context matter too. Speak to your GP for a complete picture.
What's a healthy BMI in the UK?
According to NHS guidelines, a healthy BMI is between 18.5 and 24.9 for most UK adults. For South Asian, Chinese, Black African, or Black Caribbean adults, the NHS recommends a lower range: 18.5 to 22.9. Your healthy weight range depends on your individual height.
Why are BMI thresholds different for some ethnic groups?
Research shows that adults of South Asian, Black African, and Black Caribbean heritage have higher risk of type 2 diabetes and cardiovascular disease at lower BMI levels than other populations. The NHS adjusted its BMI categories for these groups to reflect this increased health risk more accurately.
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.