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
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What Does a BMI of 12 Mean?
A BMI of 12 falls in the underweight category according to NHS guidelines. This means your weight is below the healthy range for your height. A BMI below 18.5 is classified as underweight, which can indicate insufficient body weight for optimal health. If you have a BMI of 12, it's worth speaking with your GP to understand what this means for your individual health, as underweight status can sometimes signal underlying health concerns or nutritional deficiencies. Use our free BMI calculator to check your personal BMI and explore your healthy weight range for your specific height.
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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 What Your BMI Means
The NHS BMI categories provide a standard framework for understanding whether your weight falls within a healthy range. According to NHS guidance, a BMI of 12 is well below the healthy threshold:
- Underweight: BMI below 18.5 (where a BMI of 12 falls)
- Healthy weight: BMI 18.5–24.9
- Overweight: BMI 25.0–29.9
- Obese (Class I): BMI 30.0–34.9
- Obese (Class II): BMI 35.0–39.9
- Severely obese: BMI 40.0 and above
At a BMI of 12, you are significantly underweight. The BMI formula divides your weight in kilograms by your height in metres squared. Being underweight can increase risks including weakened immunity, bone density loss, nutritional deficiencies, and hormonal imbalances. The NHS recommends discussing your underweight status with your GP, especially if it's unintentional or accompanied by symptoms like fatigue or hair 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 Weight Management Support and When to Seek Help
If you have a BMI of 12, the NHS encourages you to speak with your GP for a health assessment. Your GP can investigate whether your underweight status is due to dietary factors, medical conditions, metabolic disorders, or other health concerns. NHS guidance on BMI recognises that being significantly underweight requires attention just as much as being overweight. Your GP may refer you to a dietitian or specialist if necessary. The NHS weight management pathway typically begins in primary care, where your GP can offer personalised advice and support. Keeping a food diary and tracking your nutrition may help identify areas for improvement. Use our free weight converter to understand your weight in different units (kg, stone, lbs) as you work with your healthcare team.
- 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 of 12 dangerous?
A BMI of 12 is classified as underweight and can carry health risks. Being significantly underweight may weaken your immune system, reduce bone density, and cause nutritional deficiencies. However, context matters—your GP can assess your individual situation and advise whether your BMI poses a specific concern for your health.
What should I eat if my BMI is 12?
Focus on nutrient-dense foods including lean proteins, whole grains, healthy fats, fruits, and vegetables. Speak with your GP or a registered dietitian for personalised dietary advice tailored to your health needs. They can help identify any underlying causes of low weight.
What is the healthy BMI range according to the NHS?
The NHS defines a healthy BMI as 18.5 to 24.9 for most adults. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends a lower range of 18.5 to 22.9 due to higher health risks at lower BMI levels.
How do I calculate my BMI?
The BMI formula is weight (kg) ÷ height (m)². For example, if you weigh 45kg and are 1.60m tall, your BMI is 45 ÷ (1.60 × 1.60) = 17.6. Use our free BMI calculator on this page for gradual improvement.
Should I see my GP about a BMI of 12?
Yes. The NHS recommends consulting your GP if you have a BMI below 18.5, especially if it's unintentional or accompanied by symptoms like fatigue, weakness, or hair loss. Your GP can assess your overall health and provide appropriate support or referrals.
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.