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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Body Surface Area Equation: What It Is and How It Works
Body surface area (BSA) is a measurement used in healthcare to calculate the total external surface area of the human body, typically expressed in square metres (m²). Unlike BMI, which only considers weight and height, BSA is used in clinical settings to determine appropriate medication dosing, assess burn severity, and measure metabolic rate. Understanding BSA can provide a more complete picture of your overall health alongside other UK health metrics like your BMI. Our free BMI calculator helps you assess your weight category, whilst BSA offers another valuable clinical perspective for healthcare professionals.
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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.
Common Body Surface Area Equations
Several BSA equations are used in UK healthcare settings, each offering slightly different calculations:
- DuBois Formula (1916): BSA (m²) = 0.007184 × height (cm)^0.725 × weight (kg)^0.425 — the most widely recognised and used in NHS clinical practice
- Mosteller Formula (1987): BSA (m²) = √[height (cm) × weight (kg) ÷ 3600] — simpler to calculate and nearly as accurate as DuBois
- Haycock Formula (1978): BSA (m²) = 0.024265 × height (cm)^0.3964 × weight (kg)^0.5378 — often used in paediatric settings
- Boyd Formula (1935): BSA (m²) = 0.0003207 × height (cm)^0.3 × weight (kg)^(0.7285 - 0.0188 log weight) — less commonly used
The DuBois formula remains the gold standard in most UK healthcare settings and is embedded in NHS clinical calculator systems.
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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.
Clinical Applications and Uses
BSA is used in UK healthcare to determine appropriate dosing for chemotherapy medications, where dosage is based on m² rather than weight alone to improve accuracy and safety. It's also used to assess burn severity (percentage of BSA affected), calculate metabolic rate (energy expenditure), determine nutritional requirements in hospital settings, and guide dosing of certain biologic medications. Your GP or specialist may reference BSA in clinical assessments, particularly if you're undergoing cancer treatment or have severe burns. For general weight management and health context, our free weight converter and BMI calculator provide complementary insights into your overall health profile.
- 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 BMI and body surface area?
BMI measures weight relative to height and categorises overall weight status (underweight, healthy, overweight, obese). BSA calculates your body's actual external surface area in square metres and is primarily used for clinical medication dosing and metabolic assessments. Both metrics are useful but serve different purposes — BMI for general health screening, BSA for clinical precision. Use our free BMI calculator to check your weight category.
How do I calculate body surface area myself?
The Mosteller formula is simplest: BSA = √[height in cm × weight in kg ÷ 3600]. For example, a UK adult who is 173cm tall and weighs 70kg would calculate: √[173 × 70 ÷ 3600] = √3.36 ≈ 1.83 m². The DuBois formula is slightly more complex but more widely used in NHS settings — your healthcare provider can calculate this for you.
Why do doctors use BSA instead of just weight?
Medication effect depends on body surface area, not just weight, because larger bodies have proportionally different organ function and drug distribution. BSA accounts for height as well, making dosing safer and more accurate, particularly for chemotherapy, which can be toxic at incorrect doses. Children especially benefit from BSA-based dosing since their proportions differ from adults.
Is BSA used for weight loss medications like Mounjaro or Wegovy?
No. Weight loss medications like Mounjaro (tirzepatide) and Wegovy (semaglutide) are dosed by fixed weekly amounts (e.g. 2.5mg, 5mg, 15mg) rather than BSA-adjusted dosing. Dosing is escalated gradually based on tolerance and clinical response, not BSA calculation. Your prescriber will assess dosing during your clinical assessment.
Can I use body surface area to estimate my healthy weight?
BSA is not designed for calculating ideal body weight — that's where BMI and waist circumference are more useful. NHS healthy weight guidance is based on BMI ranges (18.5–24.9 for most adults). If you're assessing your weight management goals, our free BMI calculator and weight converter will give you clearer guidance than BSA would.
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.