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
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What Is the Ideal BMI Index According to NHS Guidelines?
The ideal body mass index for UK adults is between 18.5 and 24.9, according to NHS guidance. This range represents a healthy weight for your height and is associated with the lowest health risks. However, your ideal BMI can vary depending on your age, ethnicity, and individual health circumstances. The NHS recognises that BMI is a useful screening tool, but it has important limitations — it doesn't distinguish between muscle and fat, and doesn't account for where fat is stored on your body. Use our free BMI calculator above to check your personal BMI instantly and see where you fall on the NHS scale.
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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 Healthy Weight Ranges
The NHS defines these BMI categories for UK adults:
- Underweight: below 18.5
- Healthy weight (ideal BMI range): 18.5 to 24.9
- Overweight: 25.0 to 29.9
- Obese Class I: 30.0 to 34.9
- Obese Class II: 35.0 to 39.9
- Severely obese (Class III): 40.0 and above
For people of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower BMI thresholds. An ideal BMI for these groups starts at 18.5 but the healthy range extends only to 22.9, as these populations have a higher risk of type 2 diabetes and heart disease at the same BMI levels as other adults. Your GP can explain which thresholds apply to you. To find your healthy weight range for your height, try our weight converter tool — it calculates the exact stone and pound range that matches an ideal BMI for your measurements.
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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
If your BMI falls outside the ideal 18.5–24.9 range, the NHS offers support through several pathways. Your GP can discuss your individual health and recommend tier 1 support, which includes digital tools, community programmes, and the NHS Weight Loss Plan — all free and accessible without referral. For those with a BMI above 30, or 27 and above with weight-related conditions like type 2 diabetes or high blood pressure, specialist tier 3 weight management services may be available in your area. These services provide personalised dietary and lifestyle advice, and in some cases, prescription weight loss medications following clinical assessment. Calculate your BMI using the tool on this page, then speak with your GP about what the ideal BMI range means for your health and what support is right for you.
- 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's the difference between BMI and ideal body weight?
BMI is a screening tool based on height and weight ratio — it shows whether your weight falls in a healthy range for your height. Ideal body weight is calculated using formulas (like the Devine formula) and represents a theoretical target. The NHS primarily uses BMI ranges because they're evidence-based and linked to health outcomes. Your ideal BMI is 18.5–24.9 according to NHS guidelines, which encompasses a range of healthy weights rather than a single target.
Is an ideal BMI of 18.5–24.9 the same for everyone in the UK?
No. While 18.5–24.9 is the NHS standard for most UK adults, those of South Asian, Chinese, Black African, or Black Caribbean descent have different ideal BMI thresholds (18.5–22.9) due to higher disease risk at higher BMI levels. Age, muscle mass, and individual health conditions also affect what's ideal for you personally. Always discuss your ideal BMI with your GP, who can consider your full health picture.
Why is BMI considered ideal if it has limitations?
BMI is useful as a screening tool because it's quick, non-invasive, and strongly linked to health outcomes in populations. However, it doesn't distinguish muscle from fat — a muscular athlete might have a high BMI but low body fat. The NHS acknowledges these limitations and recommends also measuring waist circumference and considering overall health, not BMI alone. This is why talking to your GP is important alongside checking your BMI.
What should I do if my BMI is outside the ideal range?
If your BMI is below 18.5 or above 24.9, speak with your GP who can assess your individual health. Small changes in diet and physical activity often help. The NHS offers free support via tier 1 services (digital tools, community programmes). For higher BMIs, specialist weight management services and sometimes prescription medications are available through the NHS, depending on your circumstances and local services.
Can I use my healthy weight range instead of thinking about ideal BMI?
Yes — many people find it easier to think in terms of a weight range rather than a single BMI number. For your height, calculate the weight range that matches a healthy BMI of 18.5–24.9. Use our free weight converter to find your ideal weight range in stone and pounds, then track progress with our progress tracker. This can feel more practical and achievable than focusing solely on the BMI index itself.
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.