Body Surface Area Calculator

All five formulas at once — Mosteller, Du Bois, Haycock, Gehan-George and Boyd — plus how far apart they actually land, which is the part nobody publishes.

Reference table

Weight	BSA — Mosteller	Du Bois (1916)	Haycock (1978)
1	0.22 m²	0.30 m²	0.19 m²
2	0.31 m²	0.40 m²	0.27 m²
3	0.38 m²	0.47 m²	0.34 m²
4	0.43 m²	0.54 m²	0.39 m²
5	0.49 m²	0.59 m²	0.44 m²
10	0.69 m²	0.79 m²	0.64 m²
15	0.84 m²	0.94 m²	0.80 m²
20	0.97 m²	1.06 m²	0.93 m²
25	1.09 m²	1.17 m²	1.05 m²
30	1.19 m²	1.26 m²	1.16 m²
40	1.37 m²	1.43 m²	1.35 m²
50	1.54 m²	1.57 m²	1.52 m²
60	1.68 m²	1.69 m²	1.68 m²
70	1.82 m²	1.81 m²	1.83 m²
75	1.88 m²	1.86 m²	1.89 m²

📊 Typical BSA by Age

Newborn (3.5 kg, 50 cm)about 0.22 m²
Infant (8 kg, 70 cm)about 0.39 m²
Child, 6 years (20 kg, 115 cm)about 0.80 m²
Adolescent (50 kg, 160 cm)about 1.49 m²
Average adult womanabout 1.6 m²
Average adult manabout 1.9 m²

Which Formula, and When It Matters

Mosteller is the clinical default because it is a single square root and can be done at the bedside. Most chemotherapy protocols specify it.
Du Bois is the historical reference and the one most cited in papers, but it was derived from nine people — one of them a child — and it underestimates at both extremes.
Haycock and Gehan-George were both built on larger, child-inclusive samples, which is why paediatric services often prefer them.
At average adult size the five agree to within a couple of percent. In infants and in obesity the gap widens, and that is where the choice of formula changes a dose.

Why Medicine Measures People in Square Metres

Body surface area is used instead of weight for dosing whenever a drug's effect tracks metabolic rate rather than mass. Metabolic rate scales roughly with surface area, not with weight, which is why a child weighing a third of an adult does not need a third of the dose. The measure is standard in chemotherapy, where the therapeutic window is narrow enough that the difference matters, and in paediatrics for the same reason. It is also used for cardiac index — cardiac output divided by BSA — and for expressing glomerular filtration rate per 1.73 square metres, that figure being the surface area of an average adult in the studies where the convention started. Nobody measures surface area directly; every clinical figure comes from a formula built on height and weight.

Five Formulas, and What They Disagree About

Du Bois and Du Bois published theirs in 1916 from nine subjects wrapped in paper moulds, one of whom was a child, and it remains the most cited. Boyd followed in 1935 with a much larger dataset and an equation that puts the weight term inside its own exponent, which is why it is the one most often implemented incorrectly. Gehan and George in 1970 and Haycock in 1978 both used samples with far more children, and Mosteller simplified the whole thing in 1987 into a square root that gives results within a percent or two of Du Bois for ordinary adults. For someone of average size the five land within about one and a half percent of each other and the choice is immaterial. For an eight-kilogram infant, or for someone at 130 kilograms, the gap widens enough to matter — and this page shows that gap as a number rather than leaving you to compare five figures by eye.

Where the Number Is Actually Used

Chemotherapy dosing is the main use, and many protocols cap BSA at 2.0 m² regardless of the calculated value, because dosing very large patients on uncapped BSA has caused toxicity.
Paediatric dosing uses BSA for drugs where a weight-based dose would overshoot, since children have far more surface area per kilogram than adults.
Cardiac index is cardiac output divided by BSA, which is what makes outputs comparable between a small adult and a large one.
Glomerular filtration rate is normalised to 1.73 m², the average adult surface area in the original studies — not a target, just a convention.
Burns assessment uses the rule of nines or Lund-Browder charts, not these formulas. Percentage of body surface burned is a different calculation.
This is a reference tool. Any actual dose comes from the protocol, the prescriber and the pharmacy, never from a web page.

What Moves the Answer

All five formulas take only height and weight. They cannot see body composition, so a muscular and an obese person of identical height and weight get identical answers.
The disagreement between formulas grows at both extremes of size. Below about 10 kg it can exceed five percent.
Use actual measured weight, not ideal or adjusted weight, unless a specific protocol says otherwise.
For infants, length measured lying down is the right input; standing height is not measurable and not equivalent.
A one-centimetre error in height moves BSA by roughly 0.3 percent; a one-kilogram error in weight moves it by rather more.
Switching formulas mid-treatment changes the dose without anything about the patient having changed. Pick one and stay with it.

Worked Examples

Where the five agree, and where they do not

Adult, 70 kg, 170 cm — Mosteller

BSA = √(height in cm × weight in kg ÷ 3600)
= √(170 × 70 ÷ 3600)
= √3.3056

1.82 m² — the five span about 1.5%

Infant, 8 kg, 70 cm — Du Bois vs Haycock

Du Bois = 0.007184 × 8^0.425 × 70^0.725
Haycock = 0.024265 × 8^0.5378 × 70^0.3964
Same child, two equations built on different samples

The gap is several times wider than in an adult

Frequently Asked Questions

Which body surface area formula should I use?

Mosteller for most purposes: it is the clinical default, it is a single square root, and most chemotherapy protocols specify it. Paediatric services often prefer Haycock or Gehan-George because those were derived from samples with many more children. Du Bois is the historical reference but came from nine subjects and drifts at the extremes.

How much do the formulas actually differ?

For an average-sized adult, by around one to two percent — immaterial for any practical purpose. The gap widens at both extremes: in a small infant or a patient well over 100 kilograms it can reach several percent, which is enough to change a chemotherapy dose. This page shows the spread for your own figures rather than making you compare five numbers.

What is the Mosteller formula?

BSA in square metres equals the square root of height in centimetres multiplied by weight in kilograms, divided by 3600. Mosteller published it in 1987 in the New England Journal of Medicine specifically because it can be done on a bedside calculator and lands within a couple of percent of Du Bois for ordinary adults.

What is a normal body surface area?

Around 1.6 square metres for an average adult woman and around 1.9 for an average adult man. A newborn is near 0.22, a six-year-old near 0.8. The 1.73 square metres you see attached to kidney function figures is the average adult BSA from the original studies, used as a normalising convention rather than a target.

Why do chemotherapy protocols cap BSA at 2 m²?

Because dosing very large patients on uncapped surface area has produced excessive toxicity: the relationship between BSA and drug clearance flattens out at the top of the range. Many protocols therefore cap the value used for dosing, and some cap at other thresholds. It is a protocol decision, not a property of the formula.

Can I use BSA to work out a drug dose?

No. This calculator gives you the surface area; the dose comes from the prescribing protocol, the clinician and the pharmacy, and may involve capping, rounding and adjustment for organ function. Treat the number here as a reference figure, never as a dosing instruction.