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 Is a BMI Percentile and How Does It Work?
A BMI percentile compares an individual's BMI to others of the same age and sex, shown as a percentage. This measurement is crucial for children and teenagers — it's how the NHS assesses whether a young person's weight is healthy for their age and development. Unlike adults, who use fixed BMI categories, children's healthy weight ranges change as they grow, making percentiles the only accurate way to evaluate their health. Understanding BMI percentiles helps parents, carers, and healthcare professionals spot potential weight concerns early.
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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.
BMI Percentiles for Children and UK Growth Charts
The NHS uses the UK90 growth reference charts to assess children's BMI percentiles. These charts compare a child's BMI against a reference population of the same age and sex. A child's percentile position tells you where they fall relative to 100 other children of the same age and gender.
- Below 2nd centile: Underweight — may need assessment
- 2nd to 91st centile: Healthy weight range
- 91st to 98th centile: Overweight — increased health risk
- Above 98th centile: Very overweight (obese) — higher health risk
Adults use a different approach — fixed BMI formula categories (underweight, healthy weight, overweight, obese) rather than percentiles. Use our free BMI calculator above to check your personal BMI instantly, or for children, consult your GP for assessment using the UK90 charts.
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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.
Why Percentiles Matter: BMI Limitations and Practical Context
BMI percentiles account for growth and development in children, which fixed categories cannot. However, BMI — whether percentile or adult category — has important limitations. BMI does not distinguish between muscle and fat, so an athletic child with high muscle mass may appear overweight on BMI alone. Waist circumference, physical fitness level, and family history all matter too.
For children with concerns about their healthy BMI range, a GP or paediatrician should assess overall health rather than relying on BMI percentile alone. The NHS encourages balanced nutrition, regular physical activity, and family-based approaches to supporting healthy weight in young people.
- 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 percentile and adult BMI?
Children use BMI percentiles (age and sex-specific comparisons), while adults use fixed BMI categories. This is because children's healthy weight changes constantly as they grow. An adult with BMI 23 is always in the healthy range, but a 6-year-old with the same BMI might be overweight for their age.
What BMI percentile is healthy for children?
According to NHS guidance, a healthy BMI percentile for children is between the 2nd and 91st centile on UK90 growth charts. Between 2nd and 91st means the child's weight is appropriate for their age and sex. Below 2nd suggests underweight; above 91st suggests overweight.
Can I calculate my child's BMI percentile myself?
You can calculate your child's raw BMI using the standard formula (weight in kg ÷ height in metres squared), but interpreting the percentile requires UK90 growth charts, which your GP or health visitor has access to. It's best to ask your GP to assess your child's percentile position for accurate interpretation.
Why does the NHS use percentiles for children but not adults?
Children's bodies change rapidly during growth and development, so a fixed BMI threshold wouldn't be fair — a healthy weight for a 5-year-old is very different from a healthy weight for a 15-year-old. Percentiles account for this variation, whereas adults have finished growing, so fixed BMI categories work effectively.
Is BMI percentile the only way to check if my child is a healthy weight?
No. BMI percentile is a screening tool but has limitations — it doesn't distinguish between muscle and fat, and doesn't account for family history, fitness level, or overall health. Your GP can assess your child's weight holistically, considering diet, activity, growth pattern, and any health concerns alongside BMI.
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.