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 Measure BMI in the UK
Body Mass Index (BMI) is a simple measurement that helps you understand whether your weight is healthy for your height. In the UK, the NHS uses BMI as a screening tool to assess weight-related health risks. Calculating your BMI is straightforward — it takes just your height and weight — and understanding your BMI range is an important first step towards better health. This guide explains the NHS BMI formula, what the ranges mean, and when you should speak to your GP about your weight.
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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.
NHS BMI Formula and Calculation Steps
The NHS BMI formula is: weight (kg) ÷ height (m)².
To calculate your BMI:
- Measure your height in metres (or convert cm to metres by dividing by 100)
- Weigh yourself in kilograms on bathroom scales
- Multiply your height by itself
- Divide your weight by this number
For example: if you weigh 70kg and are 1.73m tall, your BMI = 70 ÷ (1.73 × 1.73) = 70 ÷ 2.99 = 23.4.
The good news is you don't need to do this manually — use our free BMI calculator above to get your exact BMI instantly. Simply enter your weight in kilograms (or convert using our weight converter if you prefer stone and pounds) and your height, and your result appears immediately with NHS guidance.
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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.
NHS BMI Categories and What They Mean
The NHS BMI categories are standardised across UK healthcare:
- Below 18.5: Underweight — your GP may recommend checking your overall health
- 18.5 to 24.9: Healthy weight — this is the target range the NHS recommends
- 25.0 to 29.9: Overweight — increased risk of weight-related health conditions
- 30.0 and above: Obese — NHS recommends speaking to your GP about weight management support
If you're of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends slightly lower thresholds because these groups face higher health risks at the same BMI levels. Always discuss your individual BMI and health with your GP, as BMI is a screening tool only and doesn't account for muscle mass or where fat is stored.
- 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.
Is BMI accurate for everyone?
BMI is a useful screening tool but has limitations. It doesn't distinguish between muscle and fat, so athletes may have a high BMI despite low body fat. BMI also doesn't account for age, bone density, or where fat is stored on your body. Your GP can assess your individual health using BMI alongside other measurements like waist circumference.
What should I do if my BMI is outside the healthy range?
If your BMI is 25 or above, speak to your GP. They can assess your overall health, check for weight-related conditions like type 2 diabetes or high blood pressure, and discuss weight management options. The NHS offers support through tier 1 programmes (self-help tools), tier 2 community services, and tier 3 specialist weight management services depending on your needs.
How often should I measure my BMI?
The NHS recommends checking your BMI periodically — typically every 3–6 months if you're actively managing your weight. Regular monitoring helps you track progress and stay motivated. Many people find our free progress tracker helpful for logging weight changes over time alongside other health markers.
Does BMI change with age?
Your BMI formula doesn't change with age — it's always weight (kg) ÷ height (m)². However, the NHS recognises that weight distribution and muscle mass change as we age. Older adults may have health risks at lower BMI levels. Always discuss your personal BMI target with your GP, as they can factor in your age, health history, and other risk factors.
What's the difference between BMI and waist measurement?
BMI measures overall weight relative to height. Waist measurement indicates where fat is stored — excess belly fat carries higher health risks. The NHS recommends both measurements: a healthy BMI (18.5–24.9) plus a waist circumference under 94cm for men or 80cm for women. Together, they give a fuller health picture than BMI alone.
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.