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's the Lowest Healthy BMI According to NHS Guidelines?
The lowest healthy BMI according to NHS guidance is 18.5. Below this figure, adults are classified as underweight, which can carry health risks just as significant as excess weight. Being underweight may indicate inadequate nutrition, underlying health conditions, or disordered eating patterns. If your BMI falls below 18.5, it's important to speak with your GP about whether your weight is appropriate for your individual health. Our free BMI calculator above can help you check your personal BMI instantly and understand where you fall within NHS categories.
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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 Threshold
The NHS BMI range divides adults into five categories, with 18.5 marking the lowest point of healthy weight:
- Below 18.5: Underweight — may indicate malnutrition or underlying health concerns
- 18.5 to 24.9: Healthy weight — the target range for most UK adults
- 25.0 to 29.9: Overweight — increased health risks begin
- 30.0 to 34.9: Obese (Class I) — significant health risks
- 35.0 and above: Severe obesity — high risk category
The BMI formula (weight in kg ÷ height in metres squared) is the same for everyone, but the thresholds reflect NHS clinical guidance on cardiovascular, metabolic, and bone health risks. An underweight BMI may signal that your body lacks essential reserves to fight infection, maintain bone density, or regulate temperature effectively. If you're underweight, your GP can assess whether dietary changes, screening for underlying conditions, or specialist referral is needed.
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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 Support and When to Seek Help
The NHS offers a tiered weight management pathway, but support isn't limited to overweight individuals — underweight adults also benefit from GP guidance. If your BMI is below 18.5, consider booking a GP appointment to discuss your weight and overall health. Your GP can check for thyroid disorders, diabetes, malabsorption conditions, or mental health factors like eating disorders that may affect your weight. Use our free weight converter to check your weight in different units (kg, stone, pounds) if that helps you understand your current position. Many GP practices also offer referrals to registered dietitians who can create a personalised nutrition plan to safely increase weight where appropriate.
- 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 always, but it warrants investigation. Some athletes and naturally lean individuals may have low BMI with excellent health. However, unintentional underweight often signals malnutrition, hormonal imbalance, or underlying illness. Your GP should assess your individual circumstances, family history, and symptoms to determine whether your weight is healthy for you.
What health risks come with being underweight?
Being underweight increases risk of weakened immune function, poor bone density (osteoporosis), muscle loss, anaemia, and fertility problems. It can also indicate eating disorders, chronic illness, or nutritional deficiency. These risks are why NHS guidance treats underweight as seriously as overweight conditions.
How can I increase my weight safely if my BMI is too low?
Speak to your GP or a registered dietitian first — they'll rule out underlying conditions and create a safe plan. Generally, eating nutrient-dense foods more frequently (nuts, avocados, olive oil, whole grains, protein) and strength training to build muscle help healthy weight gain.
What's the difference between BMI limitations for underweight versus overweight?
BMI doesn't distinguish muscle from fat at any weight level. A lean, muscular athlete might appear underweight by BMI. Conversely, someone underweight on paper may actually have poor body composition. Your GP considers BMI alongside waist circumference, fitness level, and blood work for a complete picture.
Should I use the BMI calculator if I'm concerned about being underweight?
Yes — use our free BMI calculator to get a baseline understanding of your position within NHS categories. Then share the result with your GP, who can interpret it alongside your health history, symptoms, and any underlying conditions that might explain your weight.
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.