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 Check Your BMI Index
Checking your BMI (body mass index) is a straightforward way to understand whether your weight is in a healthy range for your height. The NHS uses BMI as a key screening tool to identify whether adults are at increased health risk. This guide explains the BMI formula, how to calculate your own body mass index, and what your result means according to NHS guidelines. You can calculate your BMI manually using a simple formula, or use our free BMI calculator above for an gradual improvement.
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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.
NHS BMI Categories and What They Mean
The NHS divides adults into five BMI categories based on the standard body mass index formula: BMI = weight (kg) ÷ height (m)². These official NHS BMI categories are:
- Underweight: BMI below 18.5
- Healthy weight: BMI 18.5 to 24.9
- Overweight: BMI 25.0 to 29.9
- Obese (Class I): BMI 30.0 to 34.9
- Severely obese (Class III): BMI 40.0 and above
However, if you are 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 populations, a healthy BMI range is 18.5 to 22.9, and overweight begins at 23.0. Your GP can advise on which thresholds apply to you based on your ethnicity and health profile.
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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 Weight Management Support and When to See Your GP
If your BMI falls outside the healthy range, the NHS offers support through its structured weight management pathway. Tier 1 begins with free digital tools and community programmes — like the NHS Weight Loss Plan. If you need more support, your GP can refer you to Tier 2 or Tier 3 specialist services. You should speak to your GP if you have a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or heart disease. Your GP can assess your individual health and discuss safe, evidence-based approaches including lifestyle changes and, where appropriate, prescription weight management options.
- 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 and how do I calculate it myself?
The BMI formula is: weight (kg) ÷ height (m)². First, convert your height to metres (divide cm by 100). Square that number. Then divide your weight in kg by the squared height. For example, if you weigh 70kg and are 1.73m tall: 70 ÷ (1.73)² = 70 ÷ 2.99 = 23.4 BMI. Our free BMI calculator above will do this instantly.
What is considered a healthy BMI according to the NHS?
According to NHS guidelines, a healthy BMI for most UK adults is between 18.5 and 24.9. However, for adults of South Asian, Chinese, Black African, or Black Caribbean descent, the NHS recommends a healthy range of 18.5 to 22.9 due to higher cardiovascular risk at standard thresholds. Your GP can advise on the most appropriate range for your individual circumstances.
Does BMI tell the whole story about my health?
No — BMI has important limitations. It does not distinguish between muscle and fat, so athletes or very muscular people may have a high BMI but low body fat. BMI also ignores where fat is stored; waist circumference matters significantly. The NHS recommends considering BMI alongside other measures like waist circumference and overall health factors. Always discuss your personal health with your GP rather than focusing on BMI alone.
What should I do if my BMI is in the overweight or obese range?
If your BMI is 25 or above, speak to your GP. They can assess your overall health, discuss whether you have weight-related conditions, and recommend support — which may include dietary changes, exercise, or referral to specialist weight management services. The NHS provides free resources and structured support through its weight management pathway.
Are there different BMI ranges for children?
Yes — children's BMI is assessed very differently using age and sex-specific growth charts (UK90 centiles), not adult categories. If you're concerned about a child's weight, speak to your GP or health visitor. The NHS Healthy Weight Calculator has a separate tool for children and young people.
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.