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 Accurate Is the BMI Index?
The body mass index (BMI) is a useful screening tool recommended by the NHS, but it's not a complete picture of your health. While BMI accurately reflects weight relative to height for most people, it has important limitations—it doesn't distinguish between muscle and fat, doesn't account for where fat is stored, and may be less accurate for certain groups including athletes, older adults, and people of different ethnic backgrounds. The NHS uses BMI as a starting point but recommends considering other factors like waist circumference and overall health when assessing weight-related risk.
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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 defines BMI categories using these ranges for UK adults:
- Below 18.5 = underweight
- 18.5 to 24.9 = healthy weight
- 25.0 to 29.9 = overweight
- 30.0 to 34.9 = obese (class I)
- 35.0 to 39.9 = obese (class II)
- 40.0 and above = severely obese (class III)
However, the NHS also recognises that for people of South Asian, Chinese, Black African, and Black Caribbean heritage, lower BMI thresholds apply because these groups face higher cardiovascular and diabetes risk at lower weights. For these populations, a healthy BMI range is 18.5 to 22.9, and overweight starts at 23.0. Use our free BMI calculator above to check your personal BMI and see which NHS category you fall into. NICE guidance confirms BMI is a reasonable initial screening tool, but it should not be used in isolation for health assessment.
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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.
When BMI Is Inaccurate and What the NHS Recommends
BMI becomes significantly less accurate for athletes and muscular individuals because muscle weighs more than fat—a very fit person may have a "high" BMI but low body fat percentage. BMI is also less reliable during pregnancy, in older adults where muscle loss changes the picture, and in children where age and sex-specific charts must be used instead. The NHS therefore recommends using waist circumference alongside BMI: men with a waist over 94cm (37 inches) or women over 80cm (31.5 inches) have increased health risk, regardless of BMI. Your GP can help interpret your BMI in the context of your individual health, medical history, and other risk factors. Weight loss programmes recommended by the NHS combine dietary changes with physical activity—try our free weight converter and GLP-1 weight loss calculator to understand your current position and potential progress.
- 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 the same for everyone?
No. The NHS uses different BMI thresholds for people of South Asian, Chinese, Black African, and Black Caribbean backgrounds because they face higher disease risk at lower BMI levels. Ethnicity affects how BMI relates to health risk. Always check NHS guidance specific to your background, or ask your GP for personalised advice.
Can someone be overweight by BMI but still healthy?
Yes. If you're muscular, athletic, or have high muscle mass, you might have a BMI over 25 but low body fat and good health markers. Conversely, someone with a "healthy" BMI might have poor fitness or high visceral fat. This is why the NHS recommends using waist circumference and other factors alongside BMI.
What's the formula for calculating BMI?
BMI formula is weight (kg) divided by height in metres squared. For example, a person weighing 70kg and 1.73m tall has a BMI of 70 ÷ (1.73 × 1.73) = 23.4. Use our free BMI calculator on this page for instant calculations without doing the maths yourself.
Should I trust BMI if I'm over 40 or elderly?
BMI becomes less accurate with age because muscle naturally decreases and weight distribution changes. The NHS acknowledges this and recommends that older adults discuss BMI results with their GP in context of overall fitness, strength, and health conditions rather than relying on BMI alone.
What should I do if my BMI is in the overweight or obese range?
The NHS recommends speaking to your GP, who can assess your individual health and discuss options including lifestyle changes, structured diet programmes, and potentially prescription weight loss support if appropriate. Weight loss of just 5–10% of body weight can bring significant health benefits.
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.