Free body surface area calculator
Find your body surface area in two seconds. Enter your height and weight in metric or imperial — the calculator returns your BSA in m² using the Du Bois formula, with a Du Bois / Mosteller / Haycock comparison and the mg/m² dosing math, updated live, as you type.
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BSA is used clinically for drug dosing. Always consult a healthcare provider for medical decisions.
Results are estimates. Consult a professional.
What is body surface area (BSA)?
Body surface area (BSA) is the total surface area of the human body, expressed in square metres (m²). Because measuring the actual skin area of a living person is impractical, BSA is estimated from two numbers you already know — your height and your weight — using a validated formula. The result is a single figure that captures overall body size better than weight or height alone, which is exactly what this body surface area calculator returns the moment you enter your measurements.
BSA matters far more in clinical medicine than in everyday fitness. Many physiological quantities — metabolic rate, the volume of blood the heart pumps, kidney filtration, and the safe dose of many powerful drugs — scale with surface area rather than with body weight. A taller, heavier person has more surface area, more metabolically active tissue, and a larger circulatory system, and BSA is the single number that puts all of that on one scale.
The Du Bois BSA formula
This calculator uses the Du Bois formula, published by Du Bois and Du Bois in 1916 and still the most widely accepted standard in clinical practice. It raises height and weight to fractional powers, which is why BSA does not move in a straight line with either input — doubling your weight does not double your surface area.
The fractional exponents are the whole point. Weight carries an exponent of 0.425 and height 0.725, so surface area grows more slowly than weight and a little faster than the linear part of height. This is the mathematical reason BSA compresses the range of body sizes: a person twice as heavy has only about 35% more surface area, not twice as much.
How to calculate body surface area
Calculating BSA by hand is a short, three-step process — and the calculator above does all of it for you the instant you type. If you want to follow the maths yourself, work in metric units (centimetres and kilograms) so the constant lines up.
- Convert to metric. Height in centimetres, weight in kilograms. To convert, multiply inches by 2.54 for cm, and pounds by 0.4536 for kg.
- Apply the exponents. Raise the height to the power 0.725 and the weight to the power 0.425 — a scientific calculator's x^y button does this.
- Multiply by the constant. Multiply the two results together with 0.007184. The answer is your BSA in square metres.
If you would rather avoid fractional exponents, the Mosteller formula is far easier to do on a basic calculator: multiply height (cm) by weight (kg), divide by 3,600, and take the square root. For most adults it lands within a hundredth of a square metre of the Du Bois result, which is why it is the popular bedside shortcut.
A worked example using the body surface area calculator
Take an adult who is 180 cm (about 5 ft 11 in) tall and weighs 80 kg (about 176 lb). Here is the arithmetic the Du Bois formula runs to turn those two numbers into a BSA.
Step 1 — Raise height to the power 0.725
Take the height in centimetres and apply the exponent: 180^0.725 = 43.16.
Step 2 — Raise weight to the power 0.425
Now the weight in kilograms: 80^0.425 = 6.44.
Step 3 — Multiply by the Du Bois constant
Multiply the two results together with the constant 0.007184: 0.007184 × 43.16 × 6.44 = 1.996 m², which rounds to 2.00 m².
BSA formulas compared: Du Bois, Mosteller, and Haycock
Several formulas estimate BSA from height and weight, and they agree closely for an average-sized adult. Du Bois (1916) is the long-standing clinical standard and the one this calculator uses. Mosteller (1987) is the simplest — just a square root — and is the most common default in oncology software. Haycock (1978) was derived to better fit children and is often preferred in paediatrics. The table below runs all three on the same person.
| Formula | Equation (cm, kg) | BSA for 180 cm / 80 kg |
|---|---|---|
| Du Bois (1916) | 0.007184 × H^0.725 × W^0.425 | 2.00 m² |
| Mosteller (1987) | √(H × W ÷ 3600) | 2.00 m² |
| Haycock (1978) | 0.024265 × H^0.3964 × W^0.5378 | 2.01 m² |
H = height in cm, W = weight in kg. All three formulas computed for the same 180 cm / 80 kg adult and rounded to two decimals.
Why body surface area is used in medicine
BSA is not a vanity metric — it is a working clinical tool. Surface area tracks the size of the metabolically active body better than weight does, so several core measurements are normalised to BSA so that they can be compared across people of very different sizes.
Chemotherapy and drug dosing
The biggest use of BSA is in chemotherapy dosing. Many cancer drugs — and a number of other medications with a narrow safety margin — are prescribed in milligrams per square metre (mg/m²). The actual dose is the protocol's mg/m² figure multiplied by the patient's BSA. BSA-based dosing was adopted because surface area correlates with how the body processes and clears these drugs better than weight alone, helping keep a toxic drug inside its narrow therapeutic window across patients of different sizes.
If a protocol calls for 100 mg/m² of a drug and the patient's BSA is 2.00 m², the dose is 100 × 2.00 = 200 mg. Change the BSA to 1.6 m² and the same protocol gives 160 mg — the dose scales directly with surface area.
Cardiac index and other normalised measures
BSA is also the denominator for the cardiac index — cardiac output (litres of blood pumped per minute) divided by BSA. Indexing to surface area lets clinicians compare heart function fairly between a small adult and a large one. The same logic indexes kidney filtration (eGFR is reported per 1.73 m², a standard reference BSA), as well as some measures of lung volume and burn-injury extent.
Average body surface area values
There is no single 'normal' BSA — it rises with both height and weight. The commonly cited reference for a typical adult is about 1.7 m², and a widely used standard reference value in medicine is 1.73 m² (the figure kidney filtration is normalised to). Men average higher than women because they tend to be taller and heavier.
| Group | Typical average BSA | Common range |
|---|---|---|
| Adult (general reference) | ~1.7 m² | 1.5 – 2.0 m² |
| Adult men | ~1.9 m² | 1.7 – 2.1 m² |
| Adult women | ~1.6 m² | 1.5 – 1.8 m² |
Approximate reference values; an individual's BSA depends on their own height and weight, not their age band. Children have a much smaller BSA, which is why paediatric dosing uses BSA so carefully.
BSA vs BMI vs body weight
BSA, BMI, and body weight all use your height and weight, but they answer different questions and must not be swapped. Body weight is how much you weigh. BMI is weight relative to height (kg ÷ m²), built to screen for under- or over-weight. BSA estimates total surface area (m²), built to scale physiology and drug doses to body size.
| Metric | Unit | What it measures | Main use |
|---|---|---|---|
| Body weight | kg or lb | How much you weigh | General reference |
| BMI | kg/m² | Weight relative to height | Screening for weight category |
| BSA | m² | Total body surface area | Drug dosing, cardiac index, eGFR |
Three different metrics from the same two inputs — each built for a different job.
The practical takeaway: a high BMI flags possible excess weight, but a high BSA simply means a larger body. They can even move in opposite directions — a tall, lean person can have a large BSA but a perfectly normal BMI. For weight screening, use the BMI calculator; for body composition, the lean body mass calculator; BSA is the one to reach for when a dose or a normalised clinical measure is involved.
Limitations of BSA-based dosing
BSA is a useful estimate, but it is still an estimate, and it has well-known blind spots. Knowing them keeps the number in proper perspective.
- It is calculated, not measured. Every formula is a statistical fit to a small historical sample, so two formulas can disagree, especially at the extremes of body size.
- It ignores body composition. Two people with the same height and weight get the same BSA, even if one is muscular and the other carries more fat — so a BSA-based dose does not account for what the body is made of.
- It can mis-dose at the extremes. For very obese patients, BSA dosing can overestimate the dose because fat is less metabolically active; many protocols cap or adjust the dose, which is a clinical decision, not one this calculator can make.
- It is an old model. The classic formulas date from 1916–1987 and were derived from small samples, yet remain in use because they work well enough for most patients.
What does your BSA result mean?
Unlike BMI, BSA has no 'normal range' and no good-or-bad cut-offs. A larger BSA simply means a larger body, and a smaller BSA a smaller one — neither is healthier. So the right way to read your result is not 'is this number good?' but 'what is this number used for?'
- Below ~1.5 m² — a smaller adult or an older child. Drug doses scaled to BSA will be correspondingly lower.
- Around 1.7 m² — close to the average-adult reference figure.
- Above ~2.0 m² — a larger adult; some chemotherapy protocols cap the dose used above a BSA threshold, a clinical judgement made by the care team.
Because BSA is a clinical input rather than a fitness score, there is nothing to 'improve' about it. If your goal is body composition or weight management, BMI, body-fat percentage, and lean body mass are the metrics to track instead — BSA is the number to keep on hand when a dose or a normalised clinical measurement is involved.
Data sources and methodology
This calculator computes BSA with the Du Bois formula (Du Bois D, Du Bois EF, A formula to estimate the approximate surface area if height and weight be known, Archives of Internal Medicine, 1916), using height in centimetres and weight in kilograms. The comparison table cross-checks against the Mosteller formula (Mosteller RD, New England Journal of Medicine, 1987) and the Haycock formula (Haycock GB et al., Journal of Pediatrics, 1978). All BSA values shown are computed from these published equations and rounded for display.
Du Bois & Du Bois (1916); Mosteller (1987); Haycock et al. (1978) — published BSA formulas.Frequently asked questions about the free body surface area calculator
About this body surface area calculator
This body surface area calculator runs entirely in your browser. Your height and weight never leave your device — nothing is sent to a server, logged, or shared. It applies the Du Bois formula (BSA = 0.007184 × height^0.725 × weight^0.425, with height in cm and weight in kg) and returns your BSA in square metres, updating instantly on every change.
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