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 Does an Underweight BMI Mean in the UK?
An underweight BMI in the UK is defined as a BMI below 18.5 according to NHS guidelines. While many people focus on overweight and obesity, being significantly underweight can also affect your health and wellbeing. Understanding where your BMI falls on the NHS BMI range is the first step to assessing your personal health status. Use our free BMI calculator above to find your exact BMI and see how it compares to NHS recommendations for your height and weight.
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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 Underweight Classification
The NHS BMI categories classify adults into four main groups based on their weight and height. A BMI below 18.5 is classified as underweight, while 18.5 to 24.9 is considered a healthy weight range. The BMI formula used is weight in kilograms divided by height in metres squared. For example, an adult who is 1.70m tall and weighs 50kg would have a BMI of approximately 17.3, falling into the underweight category.
BMI limitations are important to understand: BMI does not distinguish between muscle and fat, so athletes or very muscular individuals may have a higher BMI that does not reflect excess fat. Additionally, BMI may be less accurate for elderly people, pregnant women, and those with certain medical conditions. The NHS uses BMI as a screening tool alongside other health measurements like waist circumference.
- Underweight: BMI below 18.5
- Healthy weight: BMI 18.5–24.9
- Overweight: BMI 25.0–29.9
- Obese: BMI 30.0 and above
Clinically BackedWeight Loss
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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 Guidance on Underweight Health and When to Seek Help
If your BMI calculator result shows you are underweight, the NHS recommends speaking with your GP to rule out underlying health conditions such as hyperthyroidism, malabsorption disorders, or mental health issues like depression and anxiety. Being significantly underweight can increase the risk of weakened bones (osteoporosis), weakened immune function, and nutrient deficiencies.
The NHS weight management pathway typically begins with your GP, who can assess your individual circumstances and refer you to specialist services if needed. Your GP can provide guidance on nutrition, lifestyle, and whether any medical investigations are required. If you have concerns about your weight or health, booking an appointment with your GP is the appropriate first step.
- 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.
Is a BMI below 18.5 always unhealthy?
Not necessarily. A BMI below 18.5 may be healthy for some people, especially those with naturally slight frames or high muscle mass. However, if you have recently lost weight unintentionally or have symptoms like fatigue or frequent infections, your GP should assess your health. The NHS recommends discussing any concerns about your weight with your GP for personalised advice.
What causes an underweight BMI?
Causes of low BMI include naturally small frame size, high activity levels, certain medical conditions (thyroid disorders, diabetes, digestive issues), medications, mental health conditions, inadequate nutrition, or eating disorders. Some people maintain a low BMI healthily through genetics and lifestyle. Your GP can investigate if weight loss has been unintentional or if you have other health concerns.
How accurate is the BMI formula for assessing underweight status?
The BMI formula (weight in kg ÷ height in metres²) is a useful screening tool but has limitations. It does not measure body fat directly or account for muscle mass, age, bone density, or ethnic differences in body composition. Athletes may have high BMI despite low body fat. The NHS recommends using BMI alongside waist measurement and other health markers for a complete picture.
Should I try to gain weight if my BMI is underweight?
This depends on your individual circumstances. If your low BMI is due to genetics and you feel healthy with good energy levels, weight gain may not be necessary. However, if you have unintentional weight loss, nutrient deficiencies, or health symptoms, your GP may recommend dietary changes or further investigation. Always consult your GP before making significant dietary changes.
How does the NHS BMI range differ for different ethnic groups?
The standard NHS BMI categories apply to most adults, but the NHS recommends lower BMI thresholds for people of South Asian, Chinese, Black African, and Black Caribbean heritage due to higher cardiovascular and diabetes risk at lower BMI levels. However, underweight (below 18.5) remains the same across all ethnic groups. Your GP can provide ethnicity-specific guidance if relevant to your health.
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.