BSA Calculator

What body surface area actually measures

Body surface area, or BSA, is an estimate of the total external area of the human body in square metres. Nobody measures it directly outside a research lab, so clinical practice uses regression formulas built from height and weight. The number matters because many things in medicine scale with surface area rather than with mass: heat loss, basal metabolic rate, glomerular filtration, blood volume and the clearance of a lot of drugs. Dosing a 45 kg patient and a 110 kg patient off the same flat milligram figure ignores all of that, which is why oncology, cardiology and burns units all reach for BSA.

Worked example: 170 cm and 70 kg

Take an adult of 170 cm (5 ft 7 in) and 70 kg (154 lb) and run all three formulas.

Mosteller (1987): BSA = √(height × weight / 3600) = √(170 × 70 / 3600) = √3.3056 = 1.82 m². You can do this one on a pocket calculator, which is exactly why it caught on.

Du Bois & Du Bois (1916): BSA = 0.007184 × height^0.725 × weight^0.425 = 0.007184 × 41.4 × 6.08 = 1.81 m².

Haycock (1978): BSA = 0.024265 × height^0.3964 × weight^0.5378 = 0.024265 × 7.66 × 9.82 = 1.83 m².

The three land within 0.02 m² of each other, about one percent. That is typical for adults of average build: the formulas were fitted to different populations but they all pass through roughly the same middle. The spread widens at the extremes, and that is where the choice of formula starts to matter.

Turning BSA into a dose

Chemotherapy protocols are written in mg/m². Paclitaxel at 175 mg/m² for our 1.82 m² patient works out at 175 × 1.82 = 319 mg, which pharmacy would then round to a practical vial-friendly figure. The same arithmetic runs in reverse for cardiac index: a cardiac output of 5.0 L/min divided by 1.82 m² gives 2.7 L/min/m², comfortably inside the usual 2.5 to 4.0 normal band. In burns, the Parkland formula asks for percentage of total body surface area burned, and BSA converts that percentage into the litres of crystalloid needed in the first 24 hours.

Why there are so many formulas

Du Bois and Du Bois published in 1916 after coating nine subjects in paper moulds and measuring the pieces. One of the nine was a severely malnourished child, which skewed the fit at the low end. Boyd, Gehan-George, Haycock and Mosteller followed over the next seventy years, each with a different sample and different arithmetic. Haycock's 1978 work included neonates and infants, which is why paediatric services often quote it. Mosteller published his simplification in a 1987 letter to the New England Journal of Medicine, and its square-root form is now the default in most hospital protocols precisely because it is easy to verify by hand at the bedside.

Limits worth knowing

All of these formulas assume roughly typical body proportions. At severe obesity the estimates diverge by five percent or more, and many oncology services cap BSA at 2.0 or 2.2 m² for certain agents, or dose off adjusted body weight instead, to avoid toxic overdosing. At the other end, very low weights push the exponent-based formulas around more than Mosteller. BSA also says nothing about body composition: two people at 1.9 m² can differ hugely in lean mass, and it is lean mass that clears most drugs.

Use this calculator to check a number, sanity-test a protocol calculation, or understand a figure on a clinic letter. It is a reference tool, not a prescribing tool. Any actual dose, fluid plan or haemodynamic decision belongs with the clinician who has the full picture: renal function, liver function, prior toxicity, current weight and the protocol in force where you are treated.