Body Surface Area Calculator

Mosteller: BSA = √(height × weight ÷ 3600) · Du Bois: BSA = 0.007184 × height^0.725 × weight^0.425

height

Standing height, in centimetres. Both formulas take centimetres and kilograms, which is why the numbers are what they are (cm)

weight

Body weight, in kilograms (kg)

BSA

Body surface area — what both formulas return, in square metres. An average adult lands somewhere near 1.8 (m²)

Use it when you need a size, not a judgement: a clinician has quoted a figure per square metre, a dose or an implant size asks for one, or you simply want to see what the number behind those tables is. There is no single body surface area formula, and that is the first thing this page is trying to show — the two printed here are the two most often cited, and they do not agree exactly, so a calculator that shows only one is hiding a choice it made for you. If what you actually want is an assessment of your weight, this is the wrong page: use the BMI calculator for a category or the healthy weight calculator for the distance to one, because body surface area has no range to be inside or outside of.

1. 175 cm, 70 kg

   1. Mosteller: 175 × 70 = 12250, divided by 3600 is 3.403, and the square root of that is 1.845
   2. Du Bois: 175^0.725 = 42.30, 70^0.425 = 6.084, so 0.007184 × 42.30 × 6.084 = 1.848
   3. Both round to 1.8 m², and they differ by three thousandths of a square metre

   This is the pair the two formulas agree on, and it is the reason 1.8 m² is the figure most people have seen. The agreement is not general — see the next example.

2. 150 cm, 70 kg

   1. Mosteller: 150 × 70 = 10500, divided by 3600 is 2.917, square root 1.708
   2. Du Bois: 150^0.725 = 37.82, 70^0.425 = 6.084, so 0.007184 × 37.82 × 6.084 = 1.653
   3. The same weight on a shorter frame moves the two formulas 0.06 m² apart — about three per cent

   Neither number is a mistake. The two equations were fitted to different groups of people, and they were never going to stay together at the edges of the range. If a dose depends on this figure, the formula behind it is part of the prescription.

3. 200 cm, 150 kg

   1. Mosteller: 200 × 150 = 30000, divided by 3600 is 8.333, square root 2.887
   2. Du Bois: 200^0.725 = 46.59, 150^0.425 = 8.411, so 0.007184 × 46.59 × 8.411 = 2.815
   3. Du Bois is the lower of the two here as well, but by a wider margin than at 175 cm

   Large frames are where the choice of formula costs the most in absolute terms: seven hundredths of a square metre here, against three thousandths at an ordinary adult size.

This page computes a size and stops there. It does not know what you intend to do with the number, and the two things that most often follow from a body surface area — a drug dose and a burn assessment — both need a great deal more than a height and a weight, which is why neither is computed here. A figure in square metres is not a dose: doses quoted per square metre are starting points that a clinician adjusts for kidney and liver function, for the condition being treated, and for how the drug is actually handled by that person. The formulas themselves are population fits, not measurements, so they carry an error of a few per cent on any individual, and they disagree with each other by about that much at the ends of the size range. They also assume a typical body shape for the height and weight given: marked swelling, a missing limb, a very high muscle mass or a body that does not follow the usual proportions all move the true surface area away from what any of these equations returns. None of them was fitted to replace a clinical measurement, and none of them is a reason to adjust a prescribed dose on your own.

What is body surface area?

It is a single figure, in square metres, for how large a body is — the area of its surface rather than its mass. It exists because some things in medicine scale with size rather than with weight: a dose quoted per square metre, the fraction of skin involved in a burn, or the output of the heart indexed to how big the person is. An average adult comes out near 1.8 m², a large adult near 2.2, a small child well under 1.

Which body surface area formula should I use?

If a clinician or a protocol has not named one, the Mosteller formula is the usual choice: it is the shortest of them and the only one of the three here you can work out on a pocket calculator without exponent keys. Du Bois is the older one and still appears in a lot of printed tables, so if a figure was quoted to you from such a table, Du Bois may be the one behind it. The honest answer is that at ordinary adult sizes the choice changes the second decimal place, and what matters far more is that whoever reads the number knows which formula produced it.

Why do the formulas disagree?

They were fitted to different groups of people, decades apart, using different measurements of surface area as the thing to match. A fit that is good across the middle of the population does not have to stay good at the edges, and these do not: at 175 cm and 70 kg the two answers land three thousandths of a square metre apart, while at 150 cm and 70 kg they are five hundredths apart. Neither is wrong, and the spread between them is a fair estimate of how precisely any formula can know your surface area from a height and a weight.

Is this the formula used for mg/m2 dosing?

Doses written per square metre are real and common, particularly in cancer treatment, and this figure is the one they are multiplied by. But the number this page prints is not a dose and cannot be turned into one: the mg/m2 dosing that happens in practice is set by a protocol, capped, and adjusted for organ function and for how the person actually clears the drug. If you have been given a dose, this calculator is not a second opinion on it.

Does the same formula apply to children?

The two formulas on this page are adult ones, and they were derived from adults. The Haycock formula in the chart below was built to be validated in infants and children, and paediatric doses are usually worked out from a weight-based or age-based rule rather than from a surface area at all. Small bodies are also where the formulas disagree most in relative terms, so a children's figure depends on the choice of formula far more than an adult's does.

What is a normal body surface area?

There is no normal range. Body surface area is a size, like shoe size, and a size cannot be high or low in the way a body fat percentage can — the useful comparison is against the person's own previous figure, or against what a protocol expects for their height and weight. This is why the page prints no category and no coloured badge: there is nothing here to pass or fail, and inventing a range for it would be inventing a judgement that the measurement does not contain.

• Simplified calculation of body-surface area — Mosteller RD — New England Journal of Medicine, 1987
• A formula to estimate the approximate surface area if height and weight be known (1916) — Du Bois D, Du Bois EF — Archives of Internal Medicine, 1916 (reprinted in Nutrition, 1989)
• Geometric method for measuring body surface area: a height-weight formula validated in infants, children, and adults — Haycock GB, Schwartz GJ, Wisotsky DH — Journal of Pediatrics, 1978