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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How to Calculate Your Body Mass Index — NHS Guidelines
Your body mass index (BMI) is a simple measure of your weight relative to your height. The NHS uses BMI as a screening tool to assess whether you're in a healthy weight range. Calculating your BMI is straightforward — you can do it instantly using our free BMI calculator above — but understanding what your result means in the context of NHS guidance is equally important. This guide explains the BMI formula, NHS categories, and what your BMI score tells you 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.
NHS BMI Categories and Healthy Weight Ranges
The BMI formula is simple: weight (kg) divided by height (m) squared. For example, an adult weighing 70kg and 1.73m tall would have a BMI of approximately 23.4, which falls in the healthy weight range.
The NHS uses these BMI categories for adults:
- Below 18.5: Underweight
- 18.5 to 24.9: Healthy weight (the target range)
- 25.0 to 29.9: Overweight
- 30.0 to 34.9: Obese (Class I)
- 35.0 to 39.9: Obese (Class II)
- 40.0 and above: Severely obese (Class III)
However, the NHS recognises that BMI has limitations — it doesn't distinguish between muscle and fat, so athletes may have a high BMI despite low body fat. Additionally, for people of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds, as these groups face higher health risks at lower BMI levels. Your GP can give you personalised guidance about what your BMI means for your individual health.
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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 and When to Seek Help
If your BMI falls outside the healthy range, the NHS offers a structured weight management pathway. This typically starts with self-help resources and community support programmes (Tier 1), progressing to primary care interventions if needed (Tier 2). For more significant support, your GP can refer you to specialist weight management services (Tier 3 and Tier 4), where treatment options — including prescription medications — may be discussed.
The NHS recommends losing weight gradually at a rate of 0.5 to 1 kg per week for sustainable results. Even a modest 5–10% reduction in body weight can bring meaningful health benefits, including lower blood pressure, improved blood sugar control, and reduced joint stress. Use our free weight converter to track your progress in your preferred units (kg, stone, or lbs), and speak with your GP about what a healthy weight target looks like for you.
- 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 does BMI stand for and why does the NHS use it?
BMI stands for body mass index. It's a simple screening tool the NHS uses to assess whether you're in a healthy weight range. While it has limitations — particularly for very muscular individuals — it provides a useful starting point for understanding health risks associated with excess weight. Your GP can assess your overall health beyond BMI alone.
Is there a different BMI range for people of South Asian or Black African heritage?
Yes. The NHS recognises that people of South Asian, Chinese, Black African, and Black Caribbean heritage face higher health risks at lower BMI levels. For these groups, the NHS recommends a healthy BMI range of 18.5 to 22.9 (rather than 18.5 to 24.9), and overweight status begins at BMI 23. Discuss your individual thresholds with your GP.
What's the difference between BMI and body composition?
BMI only measures weight relative to height — it doesn't show how much of your weight is muscle versus fat. Body composition is more detailed and includes waist circumference, body fat percentage, and muscle mass. The NHS also looks at waist circumference (over 102cm for men and 88cm for women indicates increased health risk) alongside BMI for a fuller picture.
How often should I check my BMI?
Many people find it helpful to check their BMI monthly or every few months as part of regular health monitoring. If you're working towards a weight loss goal, tracking your progress using our free BMI calculator and progress tracker can help you stay motivated and identify patterns over time.
Should I be concerned if my BMI is slightly above the healthy range?
A BMI slightly above 24.9 doesn't automatically mean serious health risk, especially if you're physically active and have no weight-related health conditions. However, the NHS recommends aiming for the 18.5–24.9 range long-term. Your GP can assess your overall health picture and advise whether lifestyle changes or further support would be beneficial.
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.