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 is M2 in UK Health and Weight Management?
M2 (square metres) is a measurement of body surface area (BSA) — the total external surface area of your body. In UK health contexts, M2 calculations are used by healthcare professionals to determine medication dosing, assess health risk, and plan clinical treatments. Unlike BMI, which focuses on weight relative to height, M2 gives doctors a more precise picture of your body's total surface area, which is particularly important for certain medications and specialist assessments. Understanding your M2 can help you have more informed conversations with your GP about personalised health guidance.
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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.
How M2 is Calculated and Used in UK Healthcare
Body surface area (BSA) in M2 is calculated using your height and weight. The most common UK formula is the Mosteller formula: BSA (M2) = √[(height in cm × weight in kg) ÷ 3600]. For example, an adult who is 170cm tall and weighs 75kg would have a BSA of approximately 1.88 M2.
In NHS clinical practice, M2 measurements are used for:
- Medication dosing — some chemotherapy drugs, biologics, and specialist treatments are dosed based on M2 rather than weight alone
- Health risk assessment — particularly in cardiovascular and metabolic assessments
- Renal function calculations — kidney specialists use M2 to adjust medication doses
- Nutritional assessments — especially in hospital and community dietetics
Your GP or hospital consultant can calculate your M2 as part of your health assessment. Our free BMI calculator and weight converter above help you understand your measurements in familiar UK units (stone, pounds, and kilograms), which provides important context alongside clinical M2 assessments.
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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.
M2 in Weight Management and Health Monitoring
While M2 is primarily a clinical measurement used by healthcare professionals rather than a weight management tool, understanding your body surface area can support more personalised health conversations with your GP. If you're exploring weight management options in the UK — whether through lifestyle changes, NHS weight management programmes, or prescription treatments like GLP-1 medications (Mounjaro or Wegovy) — your baseline health assessment may include M2 calculations alongside BMI and waist circumference.
The NHS uses a holistic approach to health assessment that combines multiple measurements: BMI, waist circumference, M2, and clinical history. Use our free tools to track your healthy weight range and monitor progress over time with our progress tracker. If you have questions about how M2 relates to your personal health, your GP is best placed to explain your individual assessment and recommend next steps aligned with NHS guidance.
- 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 is the difference between M2 and BMI?
BMI measures weight relative to height and is a simple screening tool. M2 (body surface area) is a more precise clinical measurement that accounts for your total external surface area and is used by healthcare professionals for medication dosing and specialist assessments. Both provide useful but different health information. Your GP can explain what each measurement means for your personal health.
Can I calculate my M2 at home?
Yes, using the Mosteller formula: √[(height in cm × weight in kg) ÷ 3600]. However, your GP or healthcare team can calculate this as part of a clinical assessment and explain what your M2 means in context. Our weight converter and BMI calculator above help you track your measurements in UK units.
Is M2 used for weight loss programmes in the UK?
M2 is not typically used to determine weight loss eligibility or targets. Instead, the NHS uses BMI, waist circumference, and health history. However, if you're prescribed specialist medications like Mounjaro or Wegovy, your baseline M2 may be recorded as part of your clinical assessment by your prescriber.
Why do some medications use M2 dosing instead of weight?
M2 dosing accounts for differences in body composition and distribution, providing more accurate medication dosing for certain treatments — particularly chemotherapy, biologics, and some specialist drugs. This helps ensure safe and effective treatment across patients of different body types and sizes.
How does M2 relate to NHS weight management services?
NHS weight management services focus on BMI, waist measurement, and health conditions rather than M2. However, if referred to specialist tier 3 or 4 services, your M2 may be calculated as part of a comprehensive health assessment to guide treatment planning and medication dosing.
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.