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 Does Body Surface Area Mean?
Body surface area (BSA) is a measurement of your total skin area, expressed in square metres (m²). It's used in healthcare to calculate medication doses, assess health risks, and understand how your body size relates to organ function. Unlike BMI, which measures weight relative to height, BSA accounts for your overall body size more comprehensively. Understanding BSA can provide additional nhs health advice context beyond standard weight measurements, particularly when discussing personalised health assessments with your GP.
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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 Is Body Surface Area Calculated?
Body surface area is typically calculated using one of two formulas. The Mosteller formula is most commonly used in UK clinical practice:
- BSA (m²) = √[height (cm) × weight (kg) ÷ 3,600]
For example, a person who is 173cm tall and weighs 75kg would have a BSA of approximately 1.88m². The DuBois formula is an alternative used in some settings. BSA measurements range from around 1.3m² for smaller adults to 2.5m² or more for larger adults. Your GP can calculate your personal BSA, or you can use our free BMI calculator above to get your weight measurement context alongside height — both are useful for understanding your overall body composition and healthy weight uk targets.
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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 Does Body Surface Area Matter for Health?
BSA is particularly important for medication dosing, especially in cancer treatment, where drugs are often prescribed per square metre of body surface rather than by simple body weight. It's also used to assess kidney function, calculate energy requirements, and evaluate cardiovascular risk. Unlike BMI alone, BSA provides a more complete picture of how your body size affects health outcomes. For weight management decisions, your doctor may consider BSA alongside BMI when assessing whether interventions are appropriate. Understanding your BSA gives you additional nhs guidance context for personalised health discussions with your healthcare provider.
- 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.
Is body surface area the same as BMI?
No. BMI measures weight relative to height (kg/m²), while BSA measures total skin surface area (m²). Both are useful health markers but calculate different things. BSA accounts for overall body size more comprehensively, whereas BMI is simpler and more widely used for weight categories in the UK.
What's a normal body surface area?
Most UK adults have a BSA between 1.5 and 2.3m². Average adult men typically range from 1.8 to 2.1m², and women from 1.6 to 1.9m². Your exact BSA depends on your individual height and weight, and there's no single "ideal" value — it's specific to your body composition.
Why do doctors use BSA for medication doses?
BSA is more accurate than body weight alone for calculating drug doses because it better reflects how medication distributes through your body and how your organs metabolise substances. This is especially important in cancer treatment and other specialist medicines where precise dosing is critical for safety.
How does body surface area relate to weight management?
BSA can help doctors assess health risk and medication suitability in weight management. Some prescription weight loss medicines may be prescribed based on your overall body size context. Your GP will consider BSA, BMI, and other health factors when recommending personalised weight management approaches.
Can I calculate my own body surface area?
Yes, using the Mosteller formula (√[height cm × weight kg ÷ 3,600]), though your GP can do this for you. You'll need accurate height and weight measurements. Our free weight converter and BMI calculator above help you establish your baseline measurements accurately before calculating BSA.
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.