BSA Calculator

Body surface area is calculated from height and weight using a formula. The most common is the Mosteller formula: BSA (m²) = √((height in cm × weight in kg) ÷ 3600). Example: 170 cm and 70 kg gives √(11900/3600) = √3.30 ≈ 1.82 m². The DuBois formula is older but still used: BSA = 0.007184 × height^0.725 × weight^0.425. Both give similar results for average adults. We use BSA every day in oncology, nephrology, and burns assessment because it correlates with metabolic rate better than weight alone.

For practical use the Mosteller formula is the most widely accepted because it's simple and accurate within 1–2% of more complex methods. DuBois and DuBois is older and slightly underestimates BSA in obese patients. Haycock works well in children. Boyd's formula handles extremes of weight better but is rarely used clinically. Clinical teams often standardize on Mosteller for medication dosing because two clinicians using the same formula get the same dose. Consistency can matter more than tiny accuracy differences in high-stakes dosing workflows.

For an average adult man, BSA is around 1.9 m². For an average adult woman, about 1.6 m². The clinical reference value used in cardiology is 1.73 m², which is why kidney function (GFR) is reported per 1.73 m². Children obviously have much smaller BSA — a newborn is around 0.25 m². Very tall or heavy people can reach 2.5 m² or more. There's no 'abnormal' BSA on its own — it's just a measurement we use to scale doses, cardiac output, and lab values to body size.

Many drugs distribute through body fluids and metabolic compartments that scale better with surface area than with weight. This is especially true for chemotherapy, where the therapeutic and toxic doses are very close. A flat per-kg dose can underdose tall thin patients and overdose short stocky ones. BSA-based dosing accounts for the relationship between metabolic rate, organ function, and body size more accurately. Pediatric drug doses, fluid resuscitation in burns (using Parkland formula), and cardiac index calculations all rely on BSA for the same reason — fairness and safety in dosing.

Yes, completely different. BMI is weight divided by height squared (kg/m²) — a ratio that screens for under- or over-weight. BSA estimates the actual area of your skin in square metres. A bodybuilder and a sedentary person of the same weight and height have the same BMI and same BSA, but very different body compositions. BMI is for population-level health screening; BSA is a clinical tool for dosing and physiology. We almost never use BMI for medication decisions and almost never use BSA for nutritional assessment.

Mosteller is the simpler one: √((height × weight) ÷ 3600). DuBois is more complex: 0.007184 × height^0.725 × weight^0.425. For most adults the two formulas agree within 0.05 m². DuBois was developed in 1916 from only 9 subjects, so it underestimates BSA in patients at the extremes — very obese or very thin. Mosteller is from 1987, easier to compute mentally, and validated more broadly. Both are acceptable, but most modern guidelines and EMR systems default to Mosteller. The key is using the same formula consistently within a patient's care.