Body Surface Area Calculator

What body surface area measures

Body surface area is the total area of a person's skin, written in square meters. A newborn is about 0.25 m2, a 10 year old about 1.15 m2, and a typical adult lands between 1.6 and 2.2 m2. The number sounds like an odd thing to compute, but it is one of the standard inputs hospitals use to scale treatments from one body to another.

The reason is that surface area tracks blood flow, heat loss and drug clearance more closely than weight does. A 250 pound adult does not need twice the drug of a 125 pound adult, because skin area grows more slowly than mass. That is why many oncology orders are written in milligrams per square meter, and why nutrition and fluid plans in burn units start from the same figure.

There is simple geometry behind the slower growth. Body volume rises with the cube of a body's linear size while surface rises with the square, so area scales roughly with weight to the power of two thirds. A body twice as heavy has about 1.6 times the surface area, not 2 times. Each published formula is a fitted version of that idea, tuned on measured patients.

One distinction worth fixing early: body surface area is a number in m2. Percent total body surface area, the number burn teams use, is a different quantity, a share of the body. Confusing the two is the most common mistake people make with this term.

Mosteller: height times weight over 3,600

The Mosteller formula, published as a letter in the New England Journal of Medicine in 1987, is the one most people mean by a body surface area calculator: BSA in m2 equals the square root of height in centimeters times weight in kilograms, divided by 3,600.

Worked example, 5 feet 10 inches and 180 pounds:

• Height: 70 inches x 2.54 = 177.8 cm
• Weight: 180 / 2.2046 = 81.6 kg
• 177.8 x 81.6 = 14,508
• 14,508 / 3,600 = 4.030
• The square root of 4.030 is 2.01 m2

The 3,600 is not a body constant. Mosteller picked it so that a single square root reproduces the older Du Bois values across the ordinary adult range, which is why the two agree within about 0.01 m2 here. The formula fits on a pocket card, needs no lookup table, and works in metric inputs only, so convert first.

A quick sanity check: an adult at 170 cm and 70 kg comes out at 1.82 m2, and most adults land somewhere between 1.5 and 2.5 m2. If a result falls outside that band, the usual cause is a unit slip, pounds entered as kilograms or inches entered as centimeters. The Mosteller letter is one page long and remains the standard reference for the formula.

Four formulas on the same body

Every published formula takes height in centimeters and weight in kilograms and returns square meters. They disagree mainly at the edges of the size range, small infants and very large adults.

H is height and W is weight. On the adult the four formulas agree inside 0.02 m2, about 1 percent. On the 40 pound child the spread is 0.03 m2, about 5 percent, and the gap keeps widening in premature infants. That is the shape of every formula family of this kind: easy agreement in the middle, disagreement at the extremes.

Du Bois and Du Bois fitted their equation in 1916 on nine subjects measured with paper molds, and it stayed the reference for most of a century. The Haycock study measured 81 subjects from premature infants to adults and found the Du Bois equation underestimated surface area by as much as 7.96 percent in newborns, which is why pediatric units often change formulas below 0.7 m2. Gehan and George worked from 401 subjects, and their equation shows up in oncology protocols.

None of the four knows what the mass is made of. Muscle, fat and fluid contribute differently to drug clearance, and the formulas cannot see any of it.

Turning BSA into a dose

With the surface area in hand, the arithmetic is one multiplication: total dose in mg equals BSA in m2 times the ordered dose in mg per m2. At 2.01 m2 and an order of 75 mg/m2, the dose is 2.01 x 75 = 150.75 mg, which a pharmacy would prepare as 151 mg; the calculator reads 150.6 mg because it multiplies the unrounded surface area.

Dose capping is the second half of the calculation. Many protocols stop raising the dose once surface area passes 2.0 m2, because the formula keeps climbing with weight while drug clearance does not keep the same pace. For the 180 pound adult the cap costs less than 1 percent of the dose. For a 6 foot 3 inch, 265 pound patient at 2.52 m2, the same 75 mg/m2 order comes to 189 mg uncapped and 150 mg capped, a 21 percent cut.

Dose banding is the other common shortcut. A protocol rounds the surface area into bands of 0.05 or 0.1 m2 and prepares a standard dose per band, which reduces preparation errors and waste. The banded dose stays within a few percent of the calculated one.

Children: 0.72 m2 is not half an adult

A 40 pound child (18.1 kg) who is 40 inches (101.6 cm) tall has a Mosteller surface area of 0.72 m2. That is 36 percent of the adult above, while the weight is 22 percent of the adult's 81.6 kg. The mismatch is the reason a pediatric dose written per m2 cannot be recovered from an adult dose by weight alone.

At 75 mg/m2 the child's dose is 0.72 x 75 = 54 mg. The same child dosed at 2 mg/kg would get 36 mg, a third less. Neither number is automatically right; the protocol decides which basis applies, and the two bases are not interchangeable.

The figures are population averages and individual children cross them at different ages, which is why the formula, not the age, drives the dose. In neonatal work the choice of formula starts to matter: for a 1.5 kg, 40 cm infant, Mosteller and Haycock both return 0.13 m2 while Du Bois returns 0.12 m2, a gap that matters when a dose is measured in tenths of a milligram. This is also why pediatric protocols name the formula rather than leaving it to whoever does the arithmetic.

Kidney function, cardiac output and the 1.73 m2 standard

Laboratories report estimated glomerular filtration rate per 1.73 m2, the surface area of a standard adult. Indexing to a fixed body lets a 1.4 m2 patient and a 2.3 m2 patient be compared on one scale. To return to the absolute filtration rate a particular body delivers, multiply the reported value by that body's surface area and divide by 1.73. At 2.01 m2 and an eGFR of 60, the absolute rate is 60 x 2.01 / 1.73 = 69.7 mL/min, and the NIDDK kidney pages explain the reporting convention in plain terms.

Cardiac index runs the same logic in the other direction: cardiac output in liters per minute, divided by surface area, gives L/min/m2 with a usual range of 2.5 to 4.0. An output of 4.5 L/min is ordinary in a 2.2 m2 adult and high in a 1.2 m2 child, and the index is what makes that visible.

Both uses rest on the same assumption, that the organ scales with skin area. It is a useful approximation with known limits, not a law of physiology, and it is why the same patient can carry one number on a lab report and a different one in a dosing protocol.

Where the number stops being reliable

Surface area formulas assume a body of ordinary proportions. They run into trouble when the shape or the composition is unusual:

• Obesity. Fat adds weight and some surface but little drug clearance, so the formula tends to overstate the dose a large body handles. Adjusted or ideal body weight, or a measured lean mass, is the usual substitute.
• Edema and fluid shifts. Extra water counts as weight the formula turns into surface area that clears nothing.
• Amputation. A missing limb removes area the formula still counts from height and weight, so results need a manual correction factor.
• Pregnancy. The added weight is maternal and fetal tissue, not a larger filtering organ.

None of these situations is rare in a hospital, which is why protocols, not formulas, decide the final dose. The National Cancer Institute's definition describes how the number is used inside oncology, and every protocol adds its own rules for capping, rounding and dose adjustments on top of it.

For a personal estimate, this calculator is a fair way to see how your size compares with a standard adult and to follow along with a written protocol. It is not a dose instruction. The treating team sets doses from the protocol, blood counts, organ function and previous tolerance, and a web page has none of that. Never change a medication based on a number from this page; that conversation belongs with your prescriber or pharmacist.