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 a BMI of 16 Mean?
A BMI of 16 falls into the underweight category according to NHS guidelines. This means your weight is below the healthy range for your height. Whilst not everyone with a low BMI has health concerns, being underweight can sometimes indicate nutritional deficiencies or underlying health issues. Our free BMI calculator above shows exactly where you sit on the NHS BMI range, and this guide explains what it means and when to speak with your GP about your weight.
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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 Your Health
The NHS uses standardised BMI categories to help assess whether weight falls within a healthy range. A BMI of 16 is classified as underweight, which sits below the healthy BMI threshold of 18.5.
- Underweight (BMI below 18.5): Weight is below the recommended range
- Healthy weight (BMI 18.5–24.9): The target range for most adults
- Overweight (BMI 25.0–29.9): Above healthy range but not obese
- Obese (BMI 30+): Significantly above healthy range
It's important to remember that the BMI formula is a screening tool only. BMI does not distinguish between muscle and fat, and it does not account for age, bone density, or individual body composition. A low BMI alone doesn't always indicate a problem, but if combined with poor nutrition or unexplained weight loss, it warrants GP assessment. Use the BMI calculator on this page to check your personal BMI and understand your individual context.
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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.
When to Speak with Your GP
If your BMI is 16, consider speaking with your GP if you experience unexplained weight loss, fatigue, poor appetite, or concerns about your nutrition. Underweight status can sometimes signal inadequate calorie intake, nutrient deficiency, or an underlying medical condition that needs attention. Your GP can assess your individual circumstances, including your medical history, lifestyle, and any symptoms. They may recommend dietary changes, refer you to a dietitian, or investigate further if needed. The NHS encourages anyone with concerns about their weight to discuss this with their healthcare team—weight management support is available through NHS services at all BMI levels.
- 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 16 healthy?
A BMI of 16 is classified as underweight by NHS standards (below 18.5). Whilst some individuals naturally have a lower BMI, this category can sometimes indicate inadequate nutrition or an underlying health issue. Your GP can assess whether your specific circumstances are healthy.
What should I weigh at my height?
Healthy weight depends on your height. Use our free BMI calculator above to find the healthy weight range for your specific height. The NHS recommends a BMI between 18.5 and 24.9 for most adults. For those of South Asian or other specific ethnicities, slightly different thresholds may apply.
Can BMI be inaccurate?
Yes. BMI does not account for muscle mass, bone density, age, or where fat is stored on your body. Athletes may have high BMI despite low body fat, whilst others may have normal BMI but poor muscle composition. BMI is a screening tool, not a complete health assessment.
What does underweight mean for my health?
Being underweight can sometimes indicate nutritional deficiency, inadequate calorie intake, or underlying illness. It may also affect bone health, immune function, and energy levels. If you're underweight with symptoms like fatigue or poor appetite, discuss this with your GP.
How can I increase my BMI healthily?
If your GP recommends weight gain, focus on nutrient-dense foods rather than empty calories. Eat regular meals with protein, healthy fats, and carbohydrates. A dietitian can create a personalised plan. Strength training may also help build muscle mass alongside modest weight gain.
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.