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 Calculator — What You Need to Know
A body surface area (BSA) calculator helps measure the total skin surface of your body, measured in square metres (m²). BSA is used in clinical practice to calculate medication doses, assess burn injuries, and evaluate kidney function. Whilst BMI remains the primary screening tool for weight status in the UK, BSA provides additional health context. Understanding your BSA alongside your BMI gives a more complete picture of your health profile. This guide explains how BSA is calculated, what it means, and how it fits into UK health assessment.
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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.
What Is Body Surface Area and Why Does It Matter?
Body surface area is the total external surface of the human body, measured in square metres (m²). Unlike weight or BMI, BSA accounts for both height and weight together in a single measurement that reflects your overall body size more accurately than weight alone.
BSA is particularly important in clinical settings because:
- Medication dosing — many cancer treatments and injectable medicines are calculated per m² of BSA rather than per kg of weight
- Kidney function assessment — estimated glomerular filtration rate (eGFR) often uses BSA adjustments
- Burn injury evaluation — percentage of BSA affected determines treatment urgency
- Cardiac assessment — BSA helps assess heart function and blood pressure targets
For weight management purposes, your GP may reference BSA alongside your BMI to get a more complete clinical picture. Use our free BMI calculator to check your BMI instantly, and combine this with BSA for fuller health context.
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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.
How Is Body Surface Area Calculated?
The most common BSA formula used in UK clinical practice is the DuBois formula:
BSA (m²) = (Height in cm × Weight in kg / 3600)^0.5
An alternative formula, the Mosteller formula, is simpler and widely used in UK hospitals:
BSA (m²) = √(Height in cm × Weight in kg / 3600)
For example, an adult who is 173 cm (5ft 8in) tall and weighs 75 kg (11st 11lbs) would have a BSA of approximately 1.9 m². Most healthy UK adults fall between 1.5 and 2.2 m². Your GP can calculate your exact BSA during a clinical assessment if needed for medication dosing or treatment planning. Try our weight converter to see your weight in different units before calculating.
- 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 in relation to height only (kg/m²), whilst BSA measures total body surface area in square metres. Both are useful but measure different things. BMI is the primary screening tool for weight status in the UK, but BSA is more relevant for medication dosing and certain clinical assessments. Use our free BMI calculator to check your BMI alongside understanding your BSA.
What is a healthy body surface area?
Most healthy UK adults have a BSA between 1.5 and 2.2 m², depending on height and weight. There is no single "healthy" BSA value — it varies widely based on individual body composition. A tall person will naturally have a larger BSA than a shorter person, even at the same weight. Your GP can advise if your BSA is appropriate for your individual health context.
Why do doctors use BSA for medication dosing?
BSA is more accurate than body weight alone for calculating medication doses because it accounts for both height and weight together. This is particularly important for chemotherapy and other potent injectable medicines where precise dosing directly affects safety and effectiveness. Your prescriber will calculate the correct dose for you based on your BSA if needed.
How does body surface area relate to weight loss?
BSA changes as your weight changes, but it is not the primary measurement for weight loss progress. Your BMI, weight in kg, and waist circumference are more relevant for tracking weight loss. If you are taking prescription weight loss medication like Mounjaro or Wegovy, your GP will monitor weight and BMI — BSA may be relevant if you later need medication dose adjustments based on your new body size.
Can I calculate my body surface area at home?
Yes, you can use the DuBois or Mosteller formula above with your height in cm and weight in kg. However, for clinical purposes (such as chemotherapy dosing), your GP or hospital should calculate it to ensure accuracy. If you need BSA for medication reasons, always ask your prescriber rather than relying on self-calculated figures.
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