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
BMI 18 — What Does It Mean?
A BMI of 18 falls comfortably within the healthy weight range according to NHS guidelines. This means your weight is considered appropriate for your height and presents a lower health risk compared to overweight or obese categories. However, BMI is just one screening tool — it doesn't account for muscle mass, bone density, or where fat is stored on your body. Your overall health depends on many factors beyond a single number. Use our free BMI calculator to check your personal BMI and see where you fall on the healthy range scale.
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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.
Where Does BMI 18 Fall on the NHS Scale?
According to NHS guidelines, BMI 18 is firmly in the healthy weight category:
- Healthy weight range: BMI 18.5 to 24.9
- BMI 18 is below 18.5 (underweight threshold), so you're actually at the lower edge of healthy
- Your weight poses a lower risk for type 2 diabetes, heart disease, and obesity-related conditions
- If you're BMI 18, you don't need to lose weight for health reasons
To calculate the BMI formula yourself: divide your weight in kilograms by your height in metres squared. For example, if you weigh 60kg and are 1.83m tall, your BMI = 60 ÷ (1.83 × 1.83) = 17.9, which rounds to 18. Our BMI calculator above does this instantly for any weight and height combination. If you're checking your healthy BMI range for your specific height, the calculator shows your personal healthy weight band in both kg and stone.
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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.
Understanding BMI Limitations and Other Health Measures
BMI has important limitations. It cannot distinguish between muscle and fat — an athlete with high muscle mass might have a BMI of 25 but very low body fat. BMI also doesn't account for age, bone density, or where fat is stored. Waist circumference is often more important than BMI alone. The NHS recommends measuring your waist: over 94cm (37 inches) for men or 80cm (31.5 inches) for women indicates increased health risk, regardless of BMI. If you're tracking weight loss progress, try our free progress tracker alongside BMI monitoring. If you have concerns about your health or weight, your GP can assess you holistically, considering fitness, diet, medical history, and other factors beyond a number.
- 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 18 considered underweight?
No. A BMI of 18 is at the lower end of the healthy weight range (18.5–24.9). However, it's very close to the underweight threshold of 18.5. For most adults, BMI 18 is perfectly healthy. If you're losing weight unintentionally, consult your GP.
What weight in stone equals a BMI of 18?
This depends on your height. For someone 1.73m (5ft 8in) tall, a BMI of 18 equals roughly 54kg (8st 7lbs). Use our weight converter and BMI calculator above to find your exact healthy weight range based on your height.
Can I trust BMI alone to assess my health?
BMI is a useful screening tool but shouldn't be your only measure. Waist circumference, fitness level, blood pressure, cholesterol, and overall lifestyle matter equally. Your GP can give you a fuller health picture than BMI alone.
Is BMI 18 the same for all ethnic backgrounds?
For most people, BMI 18 is healthy. However, the NHS recommends lower BMI thresholds for South Asian, Chinese, Black African, and Black Caribbean adults due to higher health risks at the same BMI. Your GP can advise based on your individual background and health.
Should I aim to reach a BMI of 18 if mine is higher?
Not necessarily. The entire healthy range is 18.5–24.9. Aiming for the middle of the range (around 21–22) is often more sustainable than targeting the lowest end. Focus on sustainable lifestyle changes rather than a specific BMI number. Your GP can advise what's realistic for you.
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.