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
Is BMI Reliable for Assessing Health?
BMI is a useful screening tool, but it's not a complete picture of your health. While the BMI formula provides a quick assessment based on your weight and height, it has significant limitations that mean your GP may recommend additional checks alongside it. Understanding what BMI can and cannot tell you helps you make informed decisions about your 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.
What the NHS BMI Range Actually Measures
The BMI formula calculates weight (kg) divided by height (metres) squared. The NHS uses standard BMI categories UK-wide: underweight (below 18.5), healthy weight (18.5–24.9), overweight (25.0–29.9), and obese (30 and above). However, BMI only measures the relationship between weight and height — it does not distinguish between muscle and fat. An athlete with significant muscle mass could have a high BMI despite low body fat. Additionally, BMI does not account for where fat is stored. Excess fat around the waist carries higher health risks than fat distributed elsewhere, yet BMI ignores this entirely. For South Asian, Chinese, Black African, and Black Caribbean adults, the NHS recommends lower thresholds because these populations have higher disease risk at the same BMI. Use our free BMI calculator above to check your personal BMI instantly and see which 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.
Key Limitations: Why BMI Alone Isn't Enough
BMI limitations mean doctors increasingly use complementary measures for a fuller picture. Waist circumference is a stronger predictor of cardiovascular risk — over 94cm (37 inches) for men or 80cm (31.5 inches) for women indicates increased risk. Body composition analysis, which measures actual fat percentage, is far more informative than BMI for fitness and health assessment. Blood pressure, blood sugar, cholesterol levels, and family history all matter. Elderly adults may have misleading BMI readings due to natural muscle loss with age. Pregnant women, children, and very muscular individuals cannot be accurately assessed by adult BMI categories. Your GP can assess these factors together to give you personalised health advice 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 a healthy BMI the same for everyone in the UK?
No. The NHS recommends standard thresholds for most adults (healthy BMI 18.5–24.9), but lower thresholds apply for South Asian, Chinese, Black African, and Black Caribbean people due to higher cardiovascular and diabetes risk at the same BMI. Your GP can advise on what's healthy for your ethnic background.
Can someone be overweight by BMI but still healthy?
Yes. BMI does not account for muscle mass, so athletes or very fit individuals may have a high BMI but low body fat and excellent health markers. Conversely, someone with a healthy BMI might have excess body fat and poor metabolic health, sometimes called "skinny fat."
What should I use instead of BMI to check my health?
Combine BMI with waist circumference measurements, blood pressure, blood sugar levels, cholesterol, and fitness levels for a complete picture. Your GP can order blood tests and assessments tailored to your health. Our BMI calculator and weight converter help contextualise your measurements alongside other factors.
Why do doctors still use BMI if it has limitations?
BMI is quick, free, and useful for population-level screening. It identifies potential risk in many cases. However, responsible GPs use BMI as a starting point alongside blood tests, lifestyle history, and other measurements — never in isolation — to give you accurate health advice.
Does BMI work the same way for children?
No. Children use age and sex-specific centile charts, not adult BMI categories. The NHS uses UK90 growth reference charts to assess children's weight. Always ask your GP or health visitor about your child's weight using the right assessment tool.
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.