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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Understanding BMI Percentile in UK Health Context
BMI percentile is a way of comparing your BMI against the wider population to understand how your weight compares to others of the same age and sex. In the UK, the NHS uses percentile-based BMI assessments specifically for children and young people, whilst adults are evaluated against fixed NHS BMI categories. This guide explains how percentiles work, when they're used, and what the NHS recommends for healthy weight management across all ages.
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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.
How the NHS Uses BMI Percentiles
The NHS applies BMI percentiles differently depending on your age. For children and young people aged 2–18 years, BMI is not assessed using adult categories because children's body composition changes significantly during growth. Instead, the NHS uses the UK90 growth reference charts, which measure BMI against age and sex-specific percentiles.
- Below 2nd percentile = underweight
- 2nd to 91st percentile = healthy weight
- 91st to 98th percentile = overweight
- Above 98th percentile = very overweight or obese
For adults (18+), the NHS switches to fixed BMI categories rather than percentiles: healthy weight (BMI 18.5–24.9), overweight (25–29.9), and obese (30+). This is because adult BMI thresholds are linked to specific health risks, not population distribution. The NHS also recognises that ethnicity affects BMI thresholds—South Asian, Chinese, Black African, and Black Caribbean adults face higher health risks at lower BMI levels, so lower thresholds may apply.
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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 Pathway and When to Seek Help
If your BMI falls outside the healthy range, the NHS recommends starting with Tier 1 support: self-help via the NHS Weight Loss Plan, free community weight management programmes, and lifestyle changes. Your GP can assess your individual health and may refer you to Tier 2 (primary care behaviour change support) or Tier 3 (specialist weight management services) depending on your BMI, age, and health conditions. For children, parents should speak to their GP or health visitor if they have concerns about weight. Use our free BMI calculator above to check where you stand, and discuss results with your GP for personalised guidance on next steps.
- 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 BMI category?
BMI percentile compares your BMI to the population (used for children aged 2–18 via UK90 charts). BMI category is a fixed threshold (used for adults 18+). The NHS uses percentiles for children because their body composition changes with growth, and fixed categories for adults because they correlate with specific health risks.
Why does the NHS use different BMI thresholds for South Asian and Black African adults?
Research shows these groups develop type 2 diabetes and cardiovascular disease at lower BMI levels than white European populations. The NHS recommends lower BMI thresholds (healthy range 18.5–22.9 instead of 18.5–24.9) for South Asian, Chinese, Black African, and Black Caribbean adults to better reflect their health risk profile.
My child is at the 85th percentile for BMI. Is that healthy?
If your child's BMI falls between the 2nd and 91st percentile on the UK90 growth charts, they're in the healthy weight range. The 85th percentile is well within this range. However, growth patterns matter—your health visitor or GP can assess whether the trend is appropriate for your child's individual growth pattern.
What should I do if my BMI is outside the healthy range?
Contact your GP for a health assessment. They can discuss lifestyle changes, refer you to NHS weight management support (Tier 2 or 3), or assess whether any underlying health conditions are affecting your weight. The NHS Weight Loss Plan is also available free online to get started immediately.
Does BMI percentile apply to adults in the UK?
No. The NHS uses fixed BMI categories for adults (18+), not percentiles. Adults are assessed against thresholds like healthy weight (BMI 18.5–24.9) and overweight (25–29.9). Percentiles are only used for children and young people aged 2–18 via UK90 growth reference charts.
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