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 Body Surface Area and Why Does It Matter?
Body surface area (BSA) is the total area of your skin exposed to the environment, measured in square metres (m²). It's a more accurate way to assess body size than weight alone, especially in NHS health guidance. BSA is used in clinical settings to calculate safe medication doses, assess nutritional needs, and evaluate how your body handles heat. Unlike BMI, which doesn't distinguish between muscle and fat, BSA accounts for your overall body dimensions. Understanding your BSA can give you a clearer picture of your health context beyond simple weight measurements.
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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 Body Surface Area Calculated?
BSA is calculated using your height and weight in a mathematical formula. The most widely used formula in the UK is the Mosteller formula:
- BSA (m²) = √[(height in cm × weight in kg) ÷ 3600]
For example, an adult who is 173cm tall (5ft 8in) and weighs 75kg (11st 11lbs) has a BSA of approximately 1.88 m². The DuBois formula is another common calculation, though Mosteller is simpler and preferred in many UK hospitals. BSA increases with both height and weight, but not in a straight line—taller people generally have proportionally higher BSA values. You can calculate your BSA instantly using our free BMI calculator on this page, which also gives you important health context about your weight.
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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 Do Healthcare Professionals Use Body Surface Area?
In UK NHS practice, BSA is essential for calculating safe medication doses, particularly for chemotherapy, immunosuppressants, and certain injectable treatments. BSA-based dosing is more accurate than weight-based dosing alone because it accounts for metabolic rate and drug distribution across your body. BSA is also used to assess burn severity, calculate fluid replacement needs, and evaluate nutritional status in hospitals. For weight management decisions, BSA works alongside BMI—your GP may consider both measurements when assessing your overall health. Our weight converter makes it easy to switch between UK measurements (stone and pounds) and metric (kg), helping you understand your health data clearly.
- 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 a normal body surface area for UK adults?
Average BSA for UK adults ranges from 1.6 to 2.0 m², depending on height and weight. Men typically have slightly higher BSA (1.9–2.2 m²) than women (1.6–1.9 m²) due to average height differences. Your GP can interpret your specific BSA in the context of your health.
Is body surface area the same as BMI?
No. BMI (weight ÷ height²) measures weight relative to height only. BSA measures your total skin surface area using both height and weight, making it more accurate for medication dosing and metabolic assessment. BMI is useful for screening, but BSA is more precise clinically.
Can I calculate my own body surface area at home?
Yes, using the Mosteller formula (√[(height cm × weight kg) ÷ 3600]). However, for clinical purposes—like medication dosing—your GP or hospital will calculate it. Our free BMI calculator provides health context about your weight and measurements.
Why is BSA used for medication doses instead of just weight?
Because metabolism and drug distribution don't scale linearly with weight. BSA better reflects your body's actual metabolic rate and surface exposure. This is particularly important for injectable treatments where dosing accuracy affects safety and effectiveness.
Does body surface area affect weight loss or weight management?
BSA is not directly used in weight loss decisions, but it reflects your overall body composition. Your GP will typically use BMI, waist circumference, and health history to assess weight management needs. Use our weight converter to track your measurements in UK units (stone and pounds).
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.