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 Body Mass Index (BMI)?
Body mass index (BMI) is a measure of body weight relative to height. It's calculated by dividing your weight in kilograms by your height in metres squared. The NHS uses BMI as a screening tool to assess whether you're in a healthy weight range and to identify potential health risks. While BMI has limitations — it doesn't distinguish between muscle and fat, for example — it remains a useful initial indicator of whether your weight may affect your health. Understanding your BMI is an important first step in assessing your overall wellness.
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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 Health Ranges
The NHS defines four main BMI categories for adults:
- Healthy weight: BMI 18.5 to 24.9 — the target range for most adults
- Overweight: BMI 25.0 to 29.9 — associated with increased health risks
- Obese (Class I): BMI 30.0 to 34.9 — significantly elevated health risks
- Obese (Class II or III): BMI 35.0 and above — very high health risks
However, the NHS recognises that ethnicity affects health risk at different BMI levels. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower thresholds apply: a healthy BMI is 18.5 to 22.9, and overweight begins at 23.0. This is because these populations have higher risk of type 2 diabetes and heart disease at lower BMI levels. Your GP can discuss which category is most relevant for you personally.
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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 to Seek NHS Support
If your BMI falls outside the healthy range, the NHS recommends speaking with your GP. They can assess your individual health picture, discuss lifestyle changes like diet and exercise, and determine what support might help. The NHS offers a structured weight management pathway with four tiers: self-help and digital tools (tier 1), primary care-based programmes (tier 2), specialist weight management services (tier 3), and bariatric surgery for severe obesity (tier 4). Your GP can refer you to appropriate support based on your needs. Use our free BMI calculator above to check your current BMI, then discuss the results with your healthcare provider for personalised guidance.
- 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.
How is BMI calculated?
BMI is calculated by dividing your weight in kilograms by your height in metres squared. For example, if you weigh 70kg and are 1.73m tall, your BMI would be 70 ÷ (1.73 × 1.73) = 23.4. Use our BMI calculator above to find yours instantly without a UK pharmacy calculation.
What are the limitations of BMI?
BMI doesn't account for muscle mass, which is heavier than fat. Athletes may have a high BMI but low body fat. BMI also doesn't show where fat is stored; waist circumference is important too. The NHS recommends considering BMI alongside other health markers like blood pressure and cholesterol.
Why do ethnic groups have different BMI thresholds on the NHS?
Research shows that South Asian, Chinese, Black African, and Black Caribbean adults face higher risk of diabetes and heart disease at lower BMI levels than European populations. The NHS acknowledges this by recommending a healthy BMI of 18.5–22.9 for these groups, rather than 18.5–24.9.
Should children use the same BMI categories as adults?
No. Children's BMI is assessed using age and sex-specific centile charts, not adult categories. The NHS uses UK90 growth reference standards. If you're concerned about a child's weight, speak with your GP or health visitor for appropriate assessment.
What should I do if my BMI is outside the healthy range?
Contact your GP for a health discussion. They can assess your overall health, discuss lifestyle changes, and determine if you're eligible for NHS weight management support, including specialist services or prescription weight loss treatments if appropriate.
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.