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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How to Calculate Body Surface Area — UK Health Guide
Body surface area (BSA) is a measurement that calculates the total surface area of your skin in square metres. Unlike BMI, which uses only height and weight, BSA provides a more accurate assessment for certain medical purposes, particularly in hospital and specialist settings. BSA is commonly used in dosing medications, assessing burn injuries, and monitoring metabolic rate. Understanding your BSA can give you additional health context beyond standard weight measurements available through our free BMI calculator and weight converter.
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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.
Body Surface Area Formulas and How They Work
Several formulas can calculate BSA, with the most common being the DuBois formula and the Mosteller formula. The DuBois formula uses: BSA (m²) = height (cm) × weight (kg) ÷ 3,600, then multiply by 0.007184. The Mosteller formula is simpler: BSA (m²) = √[height (cm) × weight (kg) ÷ 3,600]. Both are widely accepted in UK NHS hospitals and clinical settings.
For example, an adult who is 173 cm tall and weighs 70 kg would have a BSA of approximately 1.88 m². BSA typically ranges from 1.5 to 2.5 m² in healthy adults, though it varies based on build and body composition. Unlike BMI, which has fixed categories, BSA is individualised and compared against population averages rather than strict ranges.
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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.
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Clinical Uses of Body Surface Area in UK Healthcare
BSA is particularly important in NHS clinical practice for chemotherapy dosing, where medication amounts are calculated per square metre of body surface rather than simple body weight. It's also used in burn injury assessment (the Wallace Rule uses BSA to estimate burn severity), cardiac function monitoring, and paediatric medication dosing. Healthcare professionals may calculate BSA when assessing metabolic rate or when monitoring patients on certain medications that require body surface-based dosing. If your GP or specialist mentions BSA in your care, it's because personalised dosing or assessment based on your individual body composition is important for your safety and treatment effectiveness.
- 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 BMI and body surface area?
BMI (weight ÷ height²) measures whether your weight is healthy for your height and uses fixed categories. BSA calculates total skin surface area in square metres and is used for clinical dosing and metabolic assessment rather than weight categories. BSA accounts for overall body composition differently, making it more useful for certain medical purposes. Use our free BMI calculator to check your BMI instantly.
Can I calculate my own body surface area?
Yes, using either the DuBois or Mosteller formula with your height in centimetres and weight in kilograms. However, BSA is primarily used by healthcare professionals for clinical decisions, not for personal health monitoring. Your GP can calculate BSA as part of your medical assessment if needed for treatment purposes.
Is BSA measured in the same units across the UK?
Yes, BSA is always expressed in square metres (m²) in UK healthcare following NHS standards. Height must be in centimetres and weight in kilograms for accurate calculation using standard formulas.
Why would my doctor measure body surface area?
Your GP or hospital doctor may calculate BSA if you're receiving chemotherapy, certain medications, or specialist treatment where dosing is based on body surface rather than weight alone. It's also used in burn assessment, cardiac monitoring, and paediatric care to ensure personalised, safe treatment.
How does body surface area relate to weight loss?
BSA changes as your weight changes, but it's not a weight loss metric like BMI. If you're tracking weight loss, our GLP-1 weight loss calculator and progress tracker are more practical tools. BSA is relevant only if you're under hospital care where medication dosing depends on it.
Ready to take control of your weight loss journey?
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