Body Surface Area (BSA) Calculator
Calculate body surface area (BSA) from height and weight using common clinical formulas like Mosteller, Du Bois, Haycock, Gehan & George, and Boyd. Includes unit conversion, a formula-agreement visual that shows how much the formulas actually disagree, a simple body-size visual, and clear step-by-step output.
Background
Body surface area estimates the total external surface of the body. In clinical settings, BSA is commonly used to scale a medication dose to a person's size (a dose written as "mg per m²" gets multiplied by BSA to get an actual dose), for some chemotherapy calculations, burn assessment, and normalizing measures like cardiac index. It is an estimate — every formula here was fit from a different measured sample, so formulas typically agree within a few percent but can diverge more at less typical body sizes.
How to use this calculator
- Choose metric or imperial units.
- Enter your height and weight — for imperial height, you can fill in just feet or just inches and leave the other blank.
- Pick a primary BSA formula, or keep Mosteller as the default.
- Turn on Compare all formulas to see a visual of how much the formulas actually disagree, plus the exact numbers.
- If you know a drug's dose density (like "100 mg/m²"), enter it in the optional dose density box to see the actual calculated dose for the entered body size.
- Click Calculate to see BSA in m² and ft², plus optional BMI, formula comparison, interpretation, and steps.
How this calculator works
- The calculator first converts all inputs into centimeters and kilograms.
- Mosteller is often used because it is simple and widely taught.
- Du Bois, Haycock, Gehan & George, and Boyd are alternative formulas, each fit from a different measured sample at a different time — that's why they don't match exactly.
- The formula-agreement bars show how far each formula lands from your selected one, as a percentage — a quick way to see whether the formulas are basically agreeing or genuinely diverging for this body size.
- The calculator can also show BMI as an extra body-size reference, but BMI and BSA are different measurements: BMI compares weight to height², while BSA estimates total skin surface.
- This tool provides an estimate and should not replace clinical judgment or dosing protocols.
Formula & Equations Used
Mosteller: BSA = √((height in cm × weight in kg) / 3600)
Du Bois: BSA = 0.007184 × height^0.725 × weight^0.425
Haycock: BSA = 0.024265 × height^0.3964 × weight^0.5378
Gehan & George: BSA = 0.0235 × height^0.42246 × weight^0.51456
Boyd: BSA = 0.0003207 × height^0.3 × weight(g)^(0.7285 − 0.0188·log10(weight(g)))
BMI (optional): BMI = weight(kg) / height(m)^2
Dose scaled to BSA (context): Dose(mg) = Dose density(mg/m²) × BSA(m²)
Example Problem & Step-by-Step Solution
Example 1 — Mosteller formula
A person is 175 cm tall and weighs 72 kg. Find BSA using Mosteller.
Step: BSA = √((175×72)/3600) = √3.5.
Result: BSA ≈ 1.87 m².
Example 2 — Imperial inputs using Mosteller
A person is 5 ft 9 in tall and weighs 159 lb.
Step: Convert: 69 in × 2.54 = 175.26 cm; 159 lb ÷ 2.20462 ≈ 72.12 kg. Then BSA = √((175.26×72.12)/3600).
Result: BSA ≈ 1.87 m².
Example 3 — Pediatric example using Haycock
A child is 110 cm tall and weighs 18 kg.
Step: BSA = 0.024265 × 110^0.3964 × 18^0.5378 ≈ 0.024265 × 6.45 × 4.73.
Result: BSA ≈ 0.74 m².
Example 4 — Where formulas actually disagree
An adult is 175 cm tall and weighs 200 kg (severe obesity, an atypical body size for these formulas' original data).
Step: Mosteller gives ≈3.12 m², Du Bois ≈2.89 m², Boyd ≈3.25 m² — a spread of about 7% between the lowest and highest, versus under 1% for a typical adult body size.
Result: This is exactly why obese-patient chemotherapy dosing is a real area of clinical discussion — a 7% BSA difference becomes a real difference in a calculated dose.
Example 5 — Turning BSA into an actual dose
A drug is dosed at 100 mg/m². A patient's BSA (Mosteller) is 1.87 m².
Step: Dose = 100 mg/m² × 1.87 m².
Result: Dose ≈ 187 mg — this is the calculation BSA-based dosing is actually doing behind the scenes. Enter a dose density in the optional field above to have the calculator do this for your own numbers.
Frequently Asked Questions
What is body surface area used for?
BSA is often used to scale medication and chemotherapy doses to a person's size, for burn assessment, and for normalizing some clinical measures like cardiac index.
Which BSA formula should I use?
Mosteller is a common student-friendly default. Du Bois, Haycock, Gehan & George, and Boyd may give slightly different estimates, and Haycock is often preferred for pediatric cases.
Is BSA the same as BMI?
No. BSA estimates total body surface, while BMI compares weight to height squared as a screening measure. They describe different things and aren't interchangeable.
How much do the formulas actually disagree?
For a typical adult body size, usually well under 1%. The formula-agreement bars in the comparison panel show the real percentage for whatever height and weight you entered — it grows at less typical body sizes, like the severe-obesity example above, where it can reach around 7%.
How does BSA actually turn into a medication dose?
A drug's dosing is often written as an amount per square meter (mg/m²). Multiplying that number by a patient's BSA gives the actual dose in milligrams — see Example 5 above for a worked case.
Why do formulas give different answers at all?
Each formula was fit statistically to a different group of measured people, at a different time, using different measurement techniques — Du Bois in 1916 from just 9 people, Gehan & George in 1970 from a much larger sample, and so on. Small disagreement is the expected result of different data, not an error.
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