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 BMI Is Underweight?
A BMI below 18.5 is classified as underweight according to NHS guidelines. This means your weight is considered lower than the healthy range for your height. Underweight can occur due to various factors including genetics, medical conditions, eating disorders, or lifestyle choices. Understanding where you fall within the BMI range is important for assessing your health, though BMI is just one screening tool. Our free BMI calculator above can help you work out your personal BMI instantly and see where you sit within the NHS categories.
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 the Underweight Range
According to NHS guidelines, underweight is defined as a BMI below 18.5. The full NHS BMI range UK classification is:
- Underweight: BMI below 18.5
- Healthy weight: BMI 18.5 to 24.9
- Overweight: BMI 25.0 to 29.9
- Obese (Class I): BMI 30.0 to 34.9
- Obese (Class II): BMI 35.0 to 39.9
- Severely obese (Class III): BMI 40.0 and above
To calculate your BMI, the formula is: weight in kilograms divided by height in metres squared. For example, if you weigh 55kg and are 1.70m tall, your BMI would be approximately 19, which falls in the healthy weight range. If you weigh 50kg at the same height, your BMI would be around 17.3, putting you in the underweight category.
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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.
BMI Limitations and When to Seek Medical Advice
While BMI is a useful screening tool, it has important limitations. BMI does not distinguish between muscle and fat, so athletes or very muscular individuals may have a high BMI despite having low body fat. BMI also doesn't account for where fat is stored or changes in muscle mass with age. For underweight individuals, these limitations are equally relevant—some people may have a naturally lean frame without any health concerns.
If your BMI is underweight, consult your GP to rule out underlying medical conditions, nutritional deficiencies, or eating disorders. Your GP can provide personalised advice about whether weight gain is appropriate for you and recommend support if needed. Use our free weight converter to track your weight in kg, stone, or pounds as you monitor your health.
- 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 under 18.5 always unhealthy?
Not necessarily. Some people naturally have a lower body weight due to genetics or athletic build. However, being significantly underweight can increase risks of nutrient deficiencies, weakened immunity, and bone health issues. Your GP can assess whether your weight is healthy for you personally.
What causes someone to have an underweight BMI?
Causes include genetics, high metabolism, medical conditions (thyroid problems, digestive issues), eating disorders, stress, inadequate nutrition, or certain medications. If you're underweight without trying to be, speak to your GP to identify any underlying causes.
Can I calculate my BMI myself using the formula?
Yes. The BMI formula is: weight (kg) ÷ height (m)². For example, if you weigh 52kg and are 1.65m tall: 52 ÷ (1.65 × 1.65) = 19.1 BMI. However, our free BMI calculator on this page does this instantly and more accurately.
Does being underweight affect fertility or pregnancy?
Being significantly underweight can affect menstrual cycles and fertility in some cases, and may increase pregnancy complications. If you're underweight and planning pregnancy, discuss this with your GP or midwife before conception.
What should I do if I'm underweight?
Contact your GP for a health assessment. Don't try to gain or lose weight without professional guidance. Your GP can check for medical issues, nutrient deficiencies, and recommend appropriate dietary or lifestyle changes tailored to your individual 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.