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
BMI Stands For Body Mass Index — What You Need to Know
BMI stands for Body Mass Index — a simple measurement that helps assess whether your weight is healthy for your height. The NHS uses BMI as a screening tool to identify whether adults fall into a healthy weight range or may have increased health risks. It's calculated using a straightforward formula based on your weight in kilograms and your height in metres. Understanding what BMI means and how the NHS interprets it can help you take informed steps towards better health. Use our free BMI calculator above to check your personal BMI instantly and see where you fall on the NHS categories.
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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 Categories and What They Mean
The NHS BMI formula is: weight (kg) ÷ height (m)². The NHS divides adults into five BMI categories to help assess health risk:
- Underweight: BMI below 18.5
- Healthy weight: BMI 18.5 to 24.9 — the range the NHS recommends
- Overweight: BMI 25.0 to 29.9
- Obese (Class I): BMI 30.0 to 34.9
- Obese (Class II): BMI 35.0 to 39.9
- Severely obese (Class III): BMI 40.0 and above
It's important to note that BMI is a screening tool, not a diagnostic one. The NHS also recommends measuring waist circumference — for men, increased risk begins above 94cm (37 inches) and high risk above 102cm (40 inches); for women, increased risk begins above 80cm (31.5 inches) and high risk above 88cm (34.5 inches). Your GP can discuss what your personal BMI and waist measurement mean for your individual health.
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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 Seek Help
The NHS recognises that many adults struggle with weight management, and support is available at different levels. If your BMI falls outside the healthy range, or if you have health concerns linked to excess weight, speaking with your GP is a good first step. The NHS offers a structured weight management pathway: Tier 1 includes self-help resources and community programmes; Tier 2 provides GP-led behaviour change support; Tier 3 offers specialist multidisciplinary weight management services where prescription medications may be available; and Tier 4 covers specialist bariatric surgery services for severe obesity. Your GP can assess your situation and refer you to appropriate support. Try our weight converter to understand your weight in different units, which can help when discussing health targets with your healthcare team.
- 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 does BMI actually measure?
BMI measures the relationship between your weight and height to screen for whether you fall into a healthy weight range. It's calculated by dividing your weight in kilograms by the square of your height in metres. The NHS uses BMI as a quick assessment tool to identify potential health risks related to excess weight or being underweight.
Why does the NHS use BMI if it has limitations?
The NHS uses BMI because it's a simple, quick screening tool that works well for most adults. However, the NHS also recognises its limitations — BMI doesn't distinguish between muscle and fat, doesn't account for where fat is stored, and may be less accurate for very muscular people, elderly adults, or pregnant women. This is why your GP may discuss your waist measurement and overall health picture alongside BMI.
Are NHS BMI categories the same for all ethnic backgrounds?
No. The NHS recommends lower BMI thresholds for adults of South Asian, Chinese, Black African, and Black Caribbean heritage because these groups face higher risk of type 2 diabetes and cardiovascular disease at lower BMI levels. For these groups, a healthy BMI starts at 18.5 but the overweight threshold is 23.0 (not 25.0). Your GP can explain which thresholds apply to you.
What should I do if my BMI is outside the healthy range?
If your BMI is above 25.0, speaking with your GP is a good starting point. They can assess your individual health, discuss any weight-related concerns, and suggest appropriate support — which might include lifestyle advice, community weight management programmes, or referral to specialist services. The NHS weight management pathway offers support at different levels depending on your needs.
What's the healthy weight range for my height in the UK?
You can calculate this using your height and the NHS healthy BMI range (18.5 to 24.9). For example, at 1.73m (5ft 8in), the healthy weight range is approximately 55kg to 74kg (8st 10lbs to 11st 10lbs). Use our free BMI calculator and weight converter on this page to find your personal healthy weight range instantly.
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.