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 Reliable Is BMI for Health Assessment?
BMI (body mass index) is a useful screening tool recommended by the NHS, but it has important limitations. While BMI helps identify whether your weight falls within a healthy range, it doesn't distinguish between muscle and fat, doesn't account for where fat is stored, and may be less accurate for certain groups. The NHS recognises BMI as one part of a broader health assessment — not the whole picture. Understanding how reliable BMI actually is, and when you need additional health checks, is essential for making informed decisions about your weight and health.
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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 uses standardised BMI categories to help identify weight-related health risks in UK adults:
- 18.5–24.9: Healthy weight
- 25.0–29.9: Overweight
- 30.0–34.9: Obese (Class I)
- 35.0–39.9: Obese (Class II)
- 40.0 and above: Severely obese (Class III)
These thresholds are based on population-level health data. However, the NHS also acknowledges that BMI limitations are significant. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds because these groups face higher risks of type 2 diabetes and cardiovascular disease at lower BMI levels. Use our free BMI calculator above to check your personal BMI and see which NHS category you fall into.
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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 Unreliable — Key Limitations
BMI cannot distinguish between muscle and fat. Athletes and muscular individuals may have a high BMI but low body fat, making the formula misleading for them. BMI also ignores fat distribution — storing fat around the abdomen carries higher health risks than fat stored elsewhere, but BMI doesn't capture this. BMI is less accurate for elderly people (who naturally lose muscle) and during pregnancy. It also doesn't account for bone density, hydration status, or individual metabolic differences. The NHS recommends your GP assess your health holistically, using BMI alongside waist circumference measurements and your medical history, rather than relying on BMI alone.
- 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 accurate for muscular people?
No. BMI can't distinguish muscle from fat. A muscular person may have a high BMI but low body fat percentage. If you're very active or strength-trained, discuss your individual health risk with your GP rather than relying on BMI alone.
What's the NHS recommendation for waist measurement?
The NHS recommends measuring waist circumference alongside BMI. For men, over 94cm (37 inches) indicates increased risk; over 102cm (40 inches) indicates high risk. For women, over 80cm (31.5 inches) indicates increased risk; over 88cm (34.5 inches) indicates high risk.
Does ethnicity affect BMI thresholds?
Yes. The NHS recommends lower BMI thresholds for South Asian, Chinese, Black African, and Black Caribbean adults because these groups have higher cardiovascular and diabetes risk at lower BMI levels. Speak to your GP about thresholds relevant to your heritage.
When should I see my GP about my weight?
Contact your GP if your BMI suggests overweight or obesity, you have weight-related health symptoms, your weight has changed significantly, or you're concerned about your health. Your GP can assess you properly and discuss your individual risk factors.
How does BMI compare to body fat percentage?
BMI is a screening tool based on height and weight only. Body fat percentage is more precise but requires professional measurement. The NHS uses BMI as an initial screening tool, then may recommend additional assessments like waist circumference or referral to specialist services for detailed analysis.
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.