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 Is Surface Body Area and Why Does It Matter?
Surface body area (BSA) is a measurement of your total skin surface, calculated using your height and weight. It's an important metric used in UK healthcare for dosing medications, assessing burn severity, and evaluating nutritional needs. Unlike BMI, which only considers height and weight ratio, BSA provides a more precise reflection of body size relevant to clinical care. Understanding your BSA can help you and your GP make more informed health decisions. Use our free BMI calculator and weight converter above to check your measurements in both metric and imperial units.
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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 Is Surface Body Area Calculated?
Several formulas exist to calculate BSA, with the most common being the Mosteller formula, which is widely used in UK clinical practice:
- BSA (m²) = √[Height (cm) × Weight (kg) ÷ 3,600]
Other recognised formulas include the DuBois formula and Boyd formula, though results are broadly similar. For example, an adult who is 170cm tall and weighs 75kg would have a BSA of approximately 1.88 m². BSA is typically measured in square metres (m²) and ranges from around 1.5 m² in smaller adults to over 2.5 m² in larger individuals. NHS guidance recognises BSA as valuable for calculating safe medication doses, particularly for cancer treatments and certain systemic medicines. Your GP can calculate your personal BSA during a health assessment if clinically relevant.
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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.
UK Guidance and Next Steps
Why BSA matters in UK healthcare: BSA is particularly important for determining safe doses of chemotherapy, immunosuppressant drugs, and other potent medications where one-size-fits-all dosing would be unsafe. It's also used in assessing severe burns, calculating fluid replacement needs, and evaluating nutritional status in clinical settings. Unlike simple height-weight metrics, BSA accounts for the relationship between body size and metabolic function more accurately. For routine weight management and health screening in the UK, your BMI remains the primary tool your GP will use—but understanding BSA gives additional context for personalised healthcare discussions.
- 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 BSA and BMI?
BMI measures the ratio of weight to height squared and indicates weight category (healthy, overweight, obese). BSA measures total skin surface area and is used clinically for medication dosing and assessing body composition in specialist contexts. Both are useful but serve different purposes in UK healthcare.
Do I need to know my surface body area for weight loss?
Not typically. For weight loss management in the UK, your BMI, waist circumference, and overall health context are the primary measures your GP will use. Our free BMI calculator helps assess your weight category quickly. BSA is mainly relevant if you're undergoing medical treatment requiring dose calculations.
Is BSA used to prescribe weight loss medications like Mounjaro or Wegovy?
No. Prescription weight loss medications in the UK are prescribed based on BMI, health conditions, and clinical assessment—not BSA. Your GP or prescriber will evaluate your individual eligibility using standard NHS health guidance rather than BSA calculations.
Can surface body area change with weight loss?
Yes. BSA decreases as you lose weight, since it's directly calculated from height and weight. If you lose 10kg, your BSA will reduce accordingly. This is one reason BSA is useful in clinical settings where medication doses need adjusting during treatment.
Where can I find more information about surface body area?
Your GP can calculate your BSA if relevant to your health care. NHS guidance and clinical resources provide detailed BSA information. For general UK health guidance, use our free weight converter and BMI calculator above to understand your current measurements in context of NHS healthy weight recommendations.
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.