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 Does a BMI of 45 Mean?
A BMI of 45 places you in the severely obese category (Class III obesity) according to NHS guidelines. This BMI range indicates that excess weight may be affecting your health, and the NHS recommends seeking support. BMI is calculated using a simple formula: weight in kilograms divided by height in metres squared. However, BMI has limitations — it doesn't distinguish between muscle and fat, so it's most useful alongside other health measures like waist circumference. Your GP can provide personalised advice about what your BMI means for your individual health and what support is available.
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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 Where BMI 45 Fits
The NHS categorises BMI into clear bands to help adults understand their weight status. A BMI of 45 falls firmly in the severely obese range (BMI 40 and above), the highest category. Here's how BMI 45 compares to the full NHS scale:
- Healthy weight: BMI 18.5–24.9
- Overweight: BMI 25.0–29.9
- Obese (Class I): BMI 30.0–34.9
- Obese (Class II): BMI 35.0–39.9
- Severely obese (Class III): BMI 40.0+
At BMI 45, you're in the top severity category. The NHS recognises that at this BMI range, the health risks associated with excess weight increase significantly. These include type 2 diabetes, cardiovascular disease, high blood pressure, and joint problems. However, it's important to remember that BMI limitations mean it doesn't account for where fat is stored (visceral fat around organs is higher risk) or individual fitness levels. Your healthy BMI range depends on your height — use our free BMI calculator above to see your personal target range.
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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 offers a structured weight management pathway with support at different levels. If you have a BMI of 45, your GP may refer you to specialist weight management services (often called tier 3 or tier 4 services). These typically include dietary advice, behaviour change support, and access to prescription weight loss medications if clinically appropriate. NICE guidance supports weight loss treatment for adults with a BMI of 30 or above, particularly when combined with health conditions like type 2 diabetes or high blood pressure. Contact your GP to discuss what's available in your area — waiting lists vary by region, but specialist services are designed specifically to help with significant weight challenges. You can also start with lifestyle changes immediately: aim for a calorie deficit of around 500 kcal daily for gradual, sustainable loss.
- 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 45 dangerous?
A BMI of 45 significantly increases health risks including type 2 diabetes, heart disease, and high blood pressure. However, risk depends on many factors including age, fitness, family history, and where fat is stored. Your GP can assess your individual health and recommend support. The good news is that even modest weight loss (5–10% of body weight) can improve health outcomes.
How much weight do I need to lose to reach a healthy BMI?
This depends on your height. Use our free BMI calculator above to see your target healthy weight range. For someone 5ft 8in (173cm) tall with a BMI of 45 (approximately 110kg), reaching a healthy BMI of 24 would require losing roughly 35–40kg. Your GP or a weight management specialist can give you a personalised target.
What does the NHS recommend for someone with a BMI of 45?
The NHS recommends speaking with your GP as a first step. Depending on your health, you may be referred to specialist weight management services where you can access dietary support, behaviour change programmes, and potentially prescription medications. The NHS also recommends aiming for a loss rate of 0.5–1kg per week for sustainable results.
Can I get weight loss medication on the NHS with a BMI of 45?
Possibly, through NHS specialist weight management services. NICE guidance supports prescription weight loss treatment for people with BMI 30 or above. Access varies significantly by region and availability. Your GP can refer you to local specialist services, where a clinician will assess whether medication is appropriate for you. Waiting times can be lengthy in some areas.
How is BMI calculated, and what are its limitations?
BMI is calculated as weight (kg) divided by height (metres) squared. It's a useful screening tool but 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 elderly people or very muscular individuals. Waist circumference is also an important health measure — discuss both with your GP.
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.