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 Being Under Weighted Mean?
Being under weighted, or underweight, means your body weight falls below the healthy range for your height. According to NHS guidance, an adult with a BMI below 18.5 is classified as underweight. This can happen for various reasons — including illness, stress, insufficient calorie intake, or underlying medical conditions. While weight loss is often discussed, being significantly below a healthy weight range can also affect health. Check your personal BMI using our free BMI calculator above to understand where you fall on the healthy weight spectrum, and use our weight converter to see your weight across different UK measurements.
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.
Understanding Underweight BMI and Health Implications
An underweight BMI (below 18.5) can indicate that your body may lack adequate muscle, fat stores, and essential nutrients to function optimally. This can lead to weakened immune function, making you more susceptible to infections and slower recovery from illness. Underweight individuals may also experience reduced bone density, increasing the risk of fractures and osteoporosis later in life. Fatigue, difficulty concentrating, hormonal imbalances, and slow wound healing are common effects of being significantly underweight.
However, BMI has limitations — it does not distinguish between muscle and fat. An athletic person with high muscle mass might have a BMI below 18.5 but be perfectly healthy. Your GP can assess your individual health status beyond BMI alone. According to NHS health guidance, a healthy BMI for most UK adults falls between 18.5 and 24.9. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the healthy range starts at 18.5 to 22.9, as these populations face different health risk profiles.
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.
Weight Management and When to Seek GP Advice
If you are underweight, your GP can help identify the underlying cause — whether it is inadequate nutrition, a medical condition, medication side effects, or another factor. Weight gain should be approached carefully, focusing on nutrient-dense foods rather than empty calories. Protein intake, healthy fats, whole grains, and regular meals are key to gradual, healthy weight gain.
Do not assume you need weight loss support if you are underweight; instead, discuss your individual circumstances with your GP. They may refer you to a dietitian for personalised guidance on safe, sustainable weight gain. Our UK health information pages provide evidence-based guidance, but professional medical assessment is essential for your specific situation. Track your progress and weight changes using our free progress tracker to monitor your health journey over time.
- 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 BMI counts as being under weighted?
A BMI below 18.5 is classified as underweight according to NHS guidelines. However, BMI does not account for muscle mass, so an athletic person with low BMI might be healthy. Your GP can provide individual assessment beyond BMI alone.
Is being under weighted dangerous?
Significant underweight can increase risk of weakened immunity, reduced bone density, nutritional deficiencies, and hormonal imbalances. However, being slightly below the healthy range may not necessarily indicate a health problem. Your GP can assess whether your weight poses health concerns.
How can I gain weight healthily if I am underweight?
Focus on nutrient-dense foods including protein, healthy fats, whole grains, and regular meals. Aim for a modest calorie surplus — roughly 300–500 extra calories per day. A dietitian can create a personalised plan. Strength training can help build muscle mass alongside weight gain.
Should I use a weight converter to track underweight status?
Yes, understanding your weight in different UK measurements (kg, stone, and pounds) helps you track progress accurately. Our free weight converter makes it easy to switch between units. Combined with our BMI calculator, you can monitor both weight and health context.
When should I contact my GP about being underweight?
Contact your GP if you are significantly underweight, have unintentionally lost weight, feel unwell, lack energy, or are struggling to eat enough. Your GP can investigate causes and refer you to specialist services like dietetics if needed.
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.