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
Understanding Mass Body Index in the UK
Mass body index, commonly known as BMI, is a measurement of body weight relative to height. It's one of the most widely used tools in the UK health system to assess whether an adult's weight falls within a healthy range. The NHS uses BMI as a screening tool to identify potential health risks, though it works best alongside other measurements like waist circumference. This guide explains how BMI works, what the NHS categories mean, and how to use it as part of your overall health assessment.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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 What They Mean
The NHS classifies BMI into five main categories that help identify health risks associated with excess weight. Understanding these categories is essential for UK health guidance and weight management decisions.
- Underweight (BMI below 18.5): May indicate inadequate nutrition or underlying health conditions. Worth discussing with your GP.
- Healthy weight (BMI 18.5–24.9): The target range recommended by NHS guidance for most adults. Associated with lower risk of weight-related diseases.
- Overweight (BMI 25.0–29.9): Increases risk of type 2 diabetes, high blood pressure, and heart disease. Lifestyle changes are often recommended.
- Obese Class I (BMI 30.0–34.9): Significantly increases health risks. NHS may recommend structured weight management support.
- Obese Class II (BMI 35.0–39.9) and Class III (BMI 40.0+): Associated with serious health complications. Specialist NHS support may be available through tier 3 or tier 4 weight management services.
For adults of South Asian, Chinese, Black African, or Black Caribbean backgrounds, the NHS recommends lower thresholds because these groups face higher health risks at the same BMI level. For these populations, the healthy weight range begins at BMI 18.5–22.9, and overweight starts at BMI 23.0.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Using BMI as Part of Your UK Health Assessment
While BMI is a useful screening tool, the NHS emphasises that it has important limitations. BMI does not distinguish between muscle and fat, so athletes or very muscular individuals may have a higher BMI despite being in excellent health. It also doesn't account for where fat is stored — waist circumference is equally important. Men with a waist measurement over 94cm (37 inches) and women over 80cm (31.5 inches) face increased health risk, regardless of BMI.
Use our free BMI calculator above to check your personal BMI instantly, then combine this with a weight converter to understand your weight in different units. If you're concerned about your weight or health, your GP can provide personalised nhs health advice and discuss whether weight management support is appropriate for you. Many people benefit from combining lifestyle changes with professional support available through NHS weight management services.
- 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 is the difference between BMI and body composition?
BMI measures height-to-weight ratio only, while body composition assesses the percentage of fat, muscle, bone, and water in your body. Two people with the same BMI may have very different body compositions. Body composition is more accurate for athletes or very muscular individuals, but requires specialist measurement. BMI remains the NHS's primary screening tool because it's quick, free, and practical for population health assessment.
Why do South Asian adults have different NHS BMI thresholds?
Research shows that people of South Asian descent have higher risk of type 2 diabetes and cardiovascular disease at lower BMI levels compared to White Europeans. The NHS therefore recommends that South Asian, Chinese, Black African, and Black Caribbean adults use lower BMI thresholds for health risk assessment. This reflects their individual genetic and metabolic risk factors, not a difference in how BMI is calculated.
Can I lose weight using the healthy weight range for my height?
Yes. Calculate your healthy weight range using the NHS BMI formula — aim for a BMI between 18.5 and 24.9 (or 18.5–22.9 if you're of South Asian or other specified heritage). A safe rate of weight loss is 0.5–1kg per week, achieved through a calorie deficit of around 500 kcal daily. Our GLP-1 weight loss calculator can help you estimate potential weight loss timelines if you're considering medication-assisted approaches alongside lifestyle changes.
Should I rely only on BMI to assess my health?
No. The NHS recommends using BMI alongside waist circumference measurement for a more complete picture. Other factors matter too: blood pressure, cholesterol levels, blood sugar, family history, and lifestyle. Your GP can assess all these factors and give you personalised health guidance that goes beyond BMI alone. BMI is a useful starting point, not a complete health assessment.
How does BMI relate to weight management in the UK?
BMI helps determine whether you're eligible for NHS weight management support. Generally, adults with a BMI of 30 or above, or BMI 27–29.9 with weight-related health conditions, may access specialist NHS services or prescription weight loss options. Your GP can advise whether you meet criteria for support and what options are available in your area. Use our weight converter and BMI calculator together to understand where you currently stand.
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.