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
Understanding Body Size and Your Health
Body size refers to your overall weight, height, and how your body is proportioned — it's a key factor in health screening. In the UK, body size is typically assessed using BMI (body mass index), waist circumference, and body composition. While body size alone doesn't determine health, it's linked to risks of type 2 diabetes, heart disease, and other conditions. The good news is that even modest changes to your body size can have meaningful health benefits. Use our free BMI calculator above to understand where your current body size sits on the NHS health scale, or try our weight converter to see your measurements in different units.
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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.
How Body Size Is Measured in the UK
In the UK health system, body size is measured using several methods:
- BMI (Body Mass Index) — calculated from your weight and height. A healthy BMI is 18.5–24.9 according to NHS guidance. You can calculate your BMI instantly using our free BMI calculator on this page.
- Waist circumference — measures fat around your stomach, which is a better indicator of cardiovascular risk than BMI alone. Men should aim for under 94cm (37in); women under 80cm (31.5in).
- Body composition — the ratio of muscle to fat. Two people with the same BMI can have very different body compositions depending on muscle mass.
- Weight in different units — the NHS uses kilograms, but UK adults often think in stone and pounds. Convert between kg, stone, and lbs using our weight converter.
None of these measures is perfect alone, which is why your GP may look at several together.
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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.
Why Body Size Matters for Health
Your body size influences your health in several ways. Being overweight or obese (BMI 25 and above) increases your risk of type 2 diabetes by up to 80 times at severe obesity, alongside higher risks of heart disease, certain cancers, joint problems, and sleep apnoea. The NHS health guidance shows that even a 5–10% reduction in your body size can meaningfully improve blood pressure, blood sugar control, and cholesterol levels. You don't need to reach an "ideal" weight to see benefits — sustainable progress matters more than dramatic change. If you're concerned about your body size, your GP can discuss weight management options with you, which may include lifestyle changes, behaviour support, or prescription medication alongside diet and exercise.
- 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.
What's the difference between body size and BMI?
Body size is your overall weight and proportions; BMI is just one measurement calculated from your weight and height. BMI doesn't distinguish between muscle and fat, so it's useful as a screening tool but has limitations. Your GP may also look at waist circumference and body composition for a fuller picture of your health.
Is there an "ideal" body size?
There's no single ideal body size — it varies by genetics, muscle mass, frame size, and individual health. The NHS uses BMI ranges (18.5–24.9 is healthy) as a guide, but what matters most is feeling healthy and having good fitness and energy levels. Your GP can discuss what's realistic and healthy for you personally.
Can body size change with medication?
Yes. Prescription weight loss medications like Mounjaro and Wegovy are GLP-1 receptor agonists that may help reduce appetite and support weight loss alongside diet and exercise. In clinical trials, these medications helped people lose 14–21% of body weight on average, though individual results vary significantly. They require prescription and clinical assessment.
How quickly should body size change?
The NHS recommends losing no more than 0.5–1kg (1–2lbs) per week for sustainable results. Faster loss can lead to muscle loss and is harder to maintain long-term. A calorie deficit of around 500 calories per day typically results in about 0.5kg weekly loss. Track your progress using our free progress tracker.
Does body size affect which weight loss treatments I can access?
Yes. In the UK, prescription weight loss treatments are typically available to adults with a BMI of 30 or above, or 27 or above with weight-related health conditions like type 2 diabetes or high blood pressure. Your GP or a licensed prescriber can assess your individual eligibility and discuss suitable options.
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.