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
Body Surface Area Formula — What You Need to Know
Body surface area (BSA) is a measurement used in UK healthcare to calculate the total area of your skin, measured in square metres. Unlike BMI, which only considers weight and height, BSA provides a more accurate assessment for certain medical purposes, including medication dosing and clinical assessment. Understanding how BSA is calculated and why NHS professionals use it can help you better understand your health information and context for medical decisions.
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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 Surface Area Is Calculated in UK Healthcare
The most common formula used by NHS healthcare professionals is the Mosteller formula, which is simple and widely accepted across UK clinical practice. The formula is: BSA (m²) = √ (height in cm × weight in kg / 3600). To use this formula, take your height in centimetres and weight in kilograms, multiply them together, divide by 3600, then calculate the square root. For example, an adult who is 170cm tall and weighs 75kg would calculate: √ (170 × 75 / 3600) = √ 3.54 = 1.88 m². The DuBois formula is an older alternative, but the Mosteller formula is preferred in modern UK medical practice because it is more accurate for both adults and children.
To make this easier, you can use our free BMI calculator alongside BSA calculations—whilst BMI and BSA measure different things, understanding both gives you a complete picture of your health metrics in UK measurements.
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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 NHS Uses Body Surface Area — Clinical Applications
The NHS uses body surface area primarily for calculating medication doses, particularly in chemotherapy, immunosuppressants, and other specialist treatments where standard weight-based dosing would be inaccurate. BSA is also used to assess kidney function, calculate cardiac output, and evaluate metabolic rate in clinical research. Unlike BMI, which is a screening tool for weight-related health risks, BSA is a precise anatomical measurement. Your GP or NHS specialist may reference your BSA when discussing medication or assessing certain health conditions. If you've been given a BSA figure by your healthcare provider, it indicates they are tailoring your care based on your individual body composition rather than population averages.
- 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 BSA and BMI?
BMI measures weight in relation to height and screens for obesity-related health risks. BSA measures total skin surface area and is used for precise medication dosing and clinical assessment. Both metrics are useful in UK healthcare but for different purposes. BMI is a screening tool; BSA is a dosing calculation. You can check your BMI instantly using our free BMI calculator on this page.
Is the Mosteller formula accurate for all body types?
The Mosteller formula is accurate for most adults and children in UK clinical practice. However, it may be less precise for individuals with extreme obesity, very lean builds, or significant muscle mass. Your NHS healthcare professional will adjust calculations if needed based on your individual situation.
Do I need to calculate my own BSA?
In most cases, no—your NHS doctor, specialist, or pharmacist will calculate BSA if it's needed for your care. However, understanding the formula helps you understand why your medication dose is what it is. Always rely on NHS professionals for clinical decisions rather than self-calculated figures.
How does BSA relate to healthy weight in the UK?
BSA doesn't directly indicate whether you're a healthy weight—that's what BMI screening is for. However, both measurements together give your GP a complete picture. If you're concerned about your weight, use our BMI calculator and weight converter to understand your current metrics, then discuss with your GP about what's healthy for you individually.
Will my BSA change if I lose or gain weight?
Yes, BSA will change with significant weight changes because the formula includes weight as a variable. A 5kg weight change will slightly alter your BSA calculation. If you're on medication dosed by BSA and you lose or gain weight significantly, inform your GP or pharmacist so your dose can be reviewed if necessary.
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.