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
What Is Meant By BMI?
BMI stands for Body Mass Index — a measurement that relates your weight to your height to give a snapshot of whether you're in a healthy weight range. It's calculated using a simple formula and categorised into ranges by the NHS. BMI is a useful screening tool, but it has important limitations. In this guide, we'll explain what BMI means, how it works, the UK categories, and why your GP might discuss it with you as part of your overall health assessment.
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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.
How BMI Is Calculated and UK Categories
The BMI formula is straightforward: divide your weight in kilograms by your height in metres squared. For example, if you weigh 70kg and are 1.75m tall, your BMI would be 70 ÷ (1.75 × 1.75) = 22.9.
The NHS BMI categories for UK adults are:
- Below 18.5 = Underweight
- 18.5–24.9 = Healthy weight
- 25.0–29.9 = Overweight
- 30.0–39.9 = Obese
- 40.0+ = Severely obese
For adults 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. Your GP can explain which category applies to you and whether any action is needed based on 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.
BMI Limitations and What Else Matters
BMI is useful for population-level screening, but it has real 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 (visceral fat around the stomach is riskier than fat elsewhere) or changes that come with age. Your waist circumference is equally important: men over 94cm (37in) or women over 80cm (31.5in) have increased health risks regardless of BMI. Combined with blood pressure, cholesterol, and blood sugar levels, waist measurement gives a much fuller picture. Use our free BMI calculator above to check your BMI, then discuss the results with your GP for personalised health advice.
- 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 a BMI of 25 overweight?
Yes, according to NHS guidelines. A BMI between 25.0 and 29.9 is classified as overweight. However, BMI is just one indicator — your GP will consider your overall health, fitness level, muscle mass, and other factors before making recommendations. Use our weight converter and BMI calculator to check your current position.
What BMI should I aim for?
The NHS healthy weight range is a BMI of 18.5 to 24.9. However, the ideal BMI for you depends on your age, sex, ethnicity, fitness level, and medical history. Your GP can advise on a realistic and healthy target. For South Asian and some other backgrounds, a BMI below 23 is recommended.
Can BMI be inaccurate?
Yes. BMI doesn't account for muscle mass, bone density, age, or ethnicity-specific risk factors. Athletes, older adults, and people of certain ethnic backgrounds may have a misleading BMI reading. This is why doctors combine BMI with waist measurement, blood pressure, and blood sugar to get a complete picture.
How often should I check my BMI?
There's no strict rule, but checking every 3–6 months is reasonable if you're working on weight management. Use our free progress tracker to log your weight and trends over time. Sudden changes are worth discussing with your GP.
Does BMI matter if I feel healthy?
BMI is a screening tool, not a diagnosis. You can feel well and still have a BMI in the overweight or obese category — excess weight may not cause noticeable symptoms yet. Your GP uses BMI alongside other health checks to assess risk of future problems like diabetes or heart disease.
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.