Why is medication dosed by body surface area?

Because several physiological quantities — cardiac output, glomerular filtration, metabolic rate — scale closer to surface area than to weight. For narrow-therapeutic-index drugs, chemotherapy above all, that scaling reduces the spread of exposure across patients of different sizes. It is an approximation, and one that is actively debated in oncology.

Which formula should I use?

Mosteller, in almost all cases: it is a single square root, it is what most clinical systems implement, and it agrees with Du Bois within a couple of per cent across the normal adult range. Du Bois underestimates at the obese end, which is one of the reasons Mosteller displaced it.

What is a normal body surface area?

Around 1.7 m² for an average adult — roughly 1.6 for women and 1.9 for men — and about 0.25 m² for a newborn. Many protocols cap the value used for dosing at 2.0 or 2.2 m², because the relationship between surface area and drug clearance flattens at the top of the range.

Does body surface area account for body fat?

No, and that is its main limitation. Two people of the same height and weight get the same answer whatever their composition, while fat tissue is metabolically much less active than lean. That is why adjusted or ideal body weight is used instead for some drugs, and why the formula is a starting point rather than the whole of a dosing decision.

Body Surface Area Calculator

With weight 75 kg, height 175 cm, body surface area comes to 1.909 m² — body surface area. It is reached in 6 steps, the last of which is sqrt(175 * 75 / 3600), and each one is printed on the page with its numbers filled in. The formula is the one published by Mosteller RD. Simplified calculation of body-surface area. N Engl J Med. 1987;317(17):1098., not an approximation fitted to it.

Body surface area by the Mosteller and Du Bois formulas, the measure chemotherapy and paediatric doses are scaled by.

Formula and sources checked 27 August 2026 · How we check

Weight 75 kg, Height 175 cm

1.909 m²

Body surface area for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.

It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.

WeightHeight

Body surface area

1.909 m²

Mosteller: sqrt(175 * 75 / 3600)1.909 m²
Du Bois: 0.007184 * pow(175, 0.725) * pow(75, 0.425)1.903 m²
Haycock: 0.024265 * pow(175, 0.3964) * pow(75, 0.5378)1.917 m²
Spread between the formulas: max(1.9094065, 1.9031365, 1.9165869) - min(1.9094065, 1.9031365, 1.9165869)0.013 m²
In square feet: 1.9094065 / 0.09290320.553 sq ft
Dose at 100 mg per square meter: 1.9094065 * 100190.941 mg

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Worked example

A 75 kg, 175 cm adult has a body surface area of 1.91 m² by Mosteller and 1.90 by Du Bois — agreement to the second decimal, which is why the one-line formula replaced the older one in practice.

How to work it out yourself

1.Enter height and weight in any units the fields accept — the arithmetic runs in centimeters and kilograms.
2.Compare the formulas. Where they agree closely, as they usually do for adults, the choice does not matter; where they diverge — very small children, extremes of body composition — it says the estimate is soft.
3.Read the dose line only as an illustration of the arithmetic. Actual dosing is a clinical decision, and many protocols cap body surface area at 2.0 or 2.2 m².

The formula

Mostellersqrt(175 * 75 / 3600)
Du Bois0.007184 * pow(175, 0.725) * pow(75, 0.425)
Haycock0.024265 * pow(175, 0.3964) * pow(75, 0.5378)
Spread between the formulasmax(1.9094065, 1.9031365, 1.9165869) - min(1.9094065, 1.9031365, 1.9165869)
In square feet1.9094065 / 0.092903
Dose at 100 mg per square meter1.9094065 * 100

Source: Mosteller RD. Simplified calculation of body-surface area. N Engl J Med. 1987;317(17):1098., NIST Special Publication 811 — Guide for the Use of the International System of Units

Questions people actually ask

Why is medication dosed by body surface area?: Because several physiological quantities — cardiac output, glomerular filtration, metabolic rate — scale closer to surface area than to weight. For narrow-therapeutic-index drugs, chemotherapy above all, that scaling reduces the spread of exposure across patients of different sizes. It is an approximation, and one that is actively debated in oncology.
Which formula should I use?: Mosteller, in almost all cases: it is a single square root, it is what most clinical systems implement, and it agrees with Du Bois within a couple of per cent across the normal adult range. Du Bois underestimates at the obese end, which is one of the reasons Mosteller displaced it.
What is a normal body surface area?: Around 1.7 m² for an average adult — roughly 1.6 for women and 1.9 for men — and about 0.25 m² for a newborn. Many protocols cap the value used for dosing at 2.0 or 2.2 m², because the relationship between surface area and drug clearance flattens at the top of the range.
Does body surface area account for body fat?: No, and that is its main limitation. Two people of the same height and weight get the same answer whatever their composition, while fat tissue is metabolically much less active than lean. That is why adjusted or ideal body weight is used instead for some drugs, and why the formula is a starting point rather than the whole of a dosing decision.

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