How this works
Body Surface Area (BSA) is a clinical proxy for "how much body" there is, used mostly to dose medications that distribute by surface rather than by mass — chemotherapy is the canonical case, where doses are quoted in mg/m². It also crops up in cardiac index (cardiac output ÷ BSA), in some glomerular filtration rate adjustments, and in burn assessment by the Wallace rule of nines (which estimates burned BSA in percentage terms). For most adults BSA sits between 1.5 and 2.2 m². Four formulas dominate clinical practice. Mosteller (1987) is the simplest — √(height × weight ÷ 3600) — and the most widely used today; it agrees with the older equations within ~5% across normal body sizes and is what most modern hospital pharmacy systems use. DuBois & DuBois (1916) was the historical default for nearly a century; it slightly underestimates BSA in people with extreme body composition. Haycock (1978) was developed for paediatric use and tends to give the most accurate values in children. Boyd (1935) is sometimes used for very-low-weight adults. Differences across formulas are usually 1–4% for adults of average build but can reach 10% at the extremes (very thin, very obese, very small children). A few clinical caveats. (1) BSA underestimates surface area in obese patients, which is one reason oncology guidelines often "cap" BSA at 2.0 or 2.2 m² for chemotherapy dosing — without a cap, very obese patients would receive doses that exceed safe maxima. (2) The standard formulas assume a "normal" body habitus and don't account for amputations, severe oedema, or pregnancy. (3) For chemotherapy specifically, follow your local protocol — some regimens use unadjusted BSA, some use adjusted ideal-weight BSA, some use a flat dose. This calculator gives the raw BSA number; clinical decisions sit with the prescribing team.
The formula
H = height in cm, W = weight in kg (W_g = weight in grams for Boyd). Result is in m². Imperial inputs are converted internally before applying the formula. Mosteller is the recommended default for adults and adolescents; switch to Haycock for children, Boyd for very low body weights, DuBois for backwards compatibility with older clinical literature.
Example calculation
- Adult patient: 175 cm, 70 kg, default Mosteller formula.
- BSA = √(175 × 70 ÷ 3600) = √3.402... = 1.84 m²
- For chemotherapy dosed at 75 mg/m²: 75 × 1.84 = 138 mg total.
Frequently asked questions
Which formula should I use?
Mosteller for adult and adolescent patients — it's the modern standard, simple to compute, and what most hospital pharmacy systems use. Haycock for paediatric patients (under 18) — it was specifically validated against measured surface areas in children. Boyd for very-low-weight adults (under ~50 kg) where Mosteller can drift. DuBois only when reproducing values from older clinical literature that pre-dates Mosteller. For typical adult outpatient dosing the differences across formulas are <5% and the choice rarely changes the rounded mg total.
What's a normal BSA value?
For adults: average male around 1.9 m², average female around 1.6 m². The full normal range spans roughly 1.5–2.2 m² depending on height and build. Children scale much smaller — a newborn averages around 0.25 m², a 1-year-old around 0.5 m², a 10-year-old around 1.1 m². "Normal" here is a description, not a target — there's no clinical reason to want a particular BSA, only to know yours so dosing comes out right. Athletes, bodybuilders, and very tall people sit higher; small or thin people sit lower.
Why do hospital protocols sometimes "cap" BSA at 2.0 m²?
Two reasons. First, BSA-based dosing assumes drug clearance scales with surface area — that's roughly true for normal-weight patients but breaks down at the extremes. Very obese patients have proportionally more fat (which most drugs distribute into less than lean tissue), so a strict mg/m² dose can over-deliver active drug. Second, the standard formulas don't fully model surface area in obese patients; they extrapolate. Combining the two leads to safety guidelines that cap BSA somewhere around 2.0–2.2 m² for many cytotoxic drugs. The cap is protocol-specific — not every drug uses one — and it's set by the prescribing institution, not by the calculator.
Is this calculator suitable for actual clinical use?
The math is correct and the formulas are standard, but a calculator on a website is not a clinical decision-support system. For prescribing decisions you should use the BSA reported by your hospital's vetted EHR/pharmacy system, which has audit logs, version control, and accountability built in. This page is appropriate for: education, double-checking a value you computed by hand, planning before a clinic visit, or non-prescribing roles (research, billing, paperwork). It is not appropriate as the sole input to a chemotherapy order or any time-critical dose. As always for medical tools, this calculator does not replace the clinical judgment of a licensed practitioner familiar with your case.