Body Surface Area Calculator

How to use this tool

Choose what to work out: body surface area with all five formulas and the gap between them, cardiac index, dose per square metre as arithmetic, indexing to body surface area or to 1.73 square metres, or a table around your size. Height goes in centimetres or in feet and inches, weight in kilograms or pounds.

The input people most often get wrong is height in feet and inches. 5 ft 7 in is not 5.7 feet, which is 5 ft 8.4 in, so type 5 and 7 in the two boxes. The weight unit is next: 154 typed with kilograms selected instead of pounds turns a 1.82 m² adult at 170 cm into a 2.70 m² one, and the tool cannot catch it. This tool gives general information, not medical advice, and it is not a dosing tool.

What the calculation is doing

Body surface area is skin area in square metres, estimated from height and weight with a curve fitted to areas worked out for a group of people. Mosteller (1987) is the simplest: the square root of height in centimetres times weight in kilograms, divided by 3,600. DuBois and DuBois (1916) is 0.007184 × W^0.425 × H^0.725, Haycock (1978) is 0.024265 × W^0.5378 × H^0.3964, and Gehan and George (1970) is 0.0235 × W^0.51456 × H^0.42246. Boyd (1935) is 0.0003207 × H^0.3 × G^{(0.7285 − 0.0188 × log G)}, with G the weight in grams. Elsewhere W is kilograms and H is centimetres.

Cardiac index is output divided by area, in litres per minute per square metre. A dose is the dose per square metre times the area. An indexed value is the value divided by the area, and a value scaled to 1.73 square metres is the value times 1.73 divided by the area. Each is one multiplication or division, run for all five formulas.

Why medicine divides by surface area

The idea comes from heat. A warm body loses heat through its surface but makes it throughout its volume, and in 1883 Max Rubner reported respiration trials on dogs that supported a surface law, with metabolic rate scaling as mass to the two thirds. Karl Meeh published a surface formula in the late 19th century, 12.312 times weight to the two thirds, and the 1916 DuBois paper was the tenth in a series titled Clinical Calorimetry. StatPearls describes surface area as devised then to quantify metabolic rate and heat loss. Kleiber later found that a three quarter power fitted metabolic rates better, and the exponent is still argued over.

Medicine borrowed it for another reason. Sawyer and Ratain trace cancer drug dosing by surface area to Pinkel’s 1958 finding that doses per unit of surface area of several drugs were similar in laboratory animals and people. It was meant to pick a safe starting dose for phase I studies and was then carried into routine dosing without clear justification. Cardiac output does correlate with surface area, which is why cardiac index divides by it, but the links to renal and hepatic drug handling are weak or absent. A surface area is a scaling convention, and a convention works only if everyone uses the same one.

How far apart the five formulas are

At 170 cm and 70 kg the five results run from 1.81 to 1.83 m², a spread of 1.4%. Across 30 combinations of height from 150 to 195 cm and body mass index 20, 22.5 and 25, the spread averaged 1.7% and reached 3.9% at most, 3.0% between 160 and 190 cm. At a healthy weight the choice moves the figure by a few percent at most.

The gap grows with body mass index. At 27.5 and 30 (20 combinations) the spread averaged 3.3% and reached 6.3%. At 35 to 50 (40 combinations) it averaged 7.6% and reached 12.1%. DuBois is the low reading, 1.6% below Mosteller on average in the first band and 4.4% in the second, and the lowest of the five for all ten very large adults checked. Verbraecken and colleagues found the same in 1,868 patients in 2006, with DuBois underestimating in obesity by up to 3% in men and 5% in women. Fancher and colleagues treated a dosing gap of 4.5% or more as clinically significant and found Mosteller giving the larger dose at the 50th to 95th percentiles of US height and weight.

Small children are the other place the formulas part. At 50 cm and 3.4 kg the spread is 11.0%, at 90 cm and 13 kg 5.6%, and at 150 cm and 42 kg 1.2%. Sawyer and Ratain say DuBois came from nine patients. Haycock worked out the areas of 81 people from premature infants to adults and found DuBois underestimating below 0.7 m², by nearly 8% in newborns. Here DuBois is 6.7% below Haycock at 50 cm and 3.4 kg, less than the paper’s figure because this compares two formulas, not a formula with measured areas.

From CDC figures for 2021 to 2023, the average US woman (5 ft 3.5 in, 171.8 lb) is 1.87 m² by Mosteller and 1.82 by DuBois, a spread of 4.9%. The average man (5 ft 8.9 in, 199 lb) is 2.09 m², spread 3.2%. Compare an area with a published value only when one formula made both.

Cardiac index, indexing and the 1.73 square metre reference

Cardiac index is cardiac output divided by area. StatPearls gives the usual range as 2.5 to 4 L/min/m² and says below 2 should raise suspicion of cardiogenic shock. That range means an output of 4.3 to 6.9 L/min at 1.73 m² and 5.2 to 8.4 L/min at 2.09 m². At 175 cm and 150 kg and an output of 5 L/min the index is 1.85 by Mosteller, 1.96 by DuBois and 1.79 by Boyd.

The 1.73 m² figure is a reference, not an average. McIntosh, Möller and Van Slyke proposed it in 1928, and as Mohamed and colleagues describe, it came from the DuBois formula and 25 year old North American men and women in insurance records from 1885 to 1912. KDIGO 2024 still asks for eGFR relative to 1.73 m² in adults. UK cancer patients treated in 2005 averaged 1.91 m² for men and 1.71 m² for women by DuBois.

Scaling to 1.73 removes body size from a number, which hides it when a medicine is cleared by the kidneys a person actually has. KDIGO practice point 4.2.4 says that at extremes of body weight an eGFR not indexed for surface area may be indicated, especially for medicines with a narrow therapeutic range.

Dose per square metre is the same idea where more is at stake. At 100 mg per m² and 175 cm and 150 kg the five formulas give 256 to 279 mg. The ASCO guideline for obese adults with cancer recommends full weight based doses using actual body weight, and OncLive’s report on the 2012 version says it discouraged capping surface area at 2.0 m². Prescribing follows the institution protocol and the responsible clinician.

Body surface area by height and weight

Mosteller area in square metres, heights down the side and weights across.

Across this grid the five formulas average 4.7% apart and are furthest apart at 150 cm and 120 kg, 12.8%, a body mass index of 53. The next table puts the five side by side at six sizes.

A worked example you can check on paper

Take 170 cm and 70 kg. Mosteller: 170 × 70 = 11,900; 11,900 ÷ 3,600 = 3.3056; square root of 3.3056 = 1.8181 m². DuBois: 70 to the power 0.425 = 6.0837; 170 to the power 0.725 = 41.407; 0.007184 × 6.0837 × 41.407 = 1.8097 m². Haycock, Gehan and George and Boyd give 1.8257, 1.8313 and 1.8347 m². A published check: Medical Mathematics by Mike LePine gets 1.84 m² by Mosteller at 175 cm and 70 kg, and the tool returns the same.

Output of 5 L/min: 5 ÷ 1.8181 = 2.750 L/min/m². A dose of 100 mg per m², as arithmetic: 100 × 1.8181 = 181.8 mg. A value of 100 scaled to 1.73 m²: 100 × 1.73 ÷ 1.8181 = 95.15.

Mistakes that catch people out

Typing feet as a decimal. 5.7 feet is 5 ft 8.4 in. Pick feet and inches and enter the inches in their own box.

Leaving the weight unit on kilograms. A 154 lb adult typed as 154 kg at 170 cm is 2.70 m², not 1.82 m². Check the unit beside the weight.

Mixing formulas between sources. An area, index or dose compares only with one from the same formula. Switching a very large adult from Mosteller to DuBois moves the area by 5.4%.

Treating 1.73 as the average person. Measured averages run from 1.71 m² for UK women to 2.09 m² for US men.

Rounding twice. Round the area once, by the rule your protocol names, and only then multiply.

Popular calculations

Each opens the calculator with those figures filled in.

Body surface area for adults

Infants and children, Haycock first

Very large adults and US averages

Cardiac index

Dose per square metre, arithmetic only

Indexing and the 1.73 m² reference

Tables of body surface area

How this was checked

The formulas match their published forms: the DuBois 1916 form with 71.84 to 1e−16, the metre form with 0.20247 to 1.3e−5, and Boyd’s kilogram form to 0.010%. An independent implementation using exponentials and logarithms agreed on 100,000 evaluations to 3.3e−15. The Medical Mathematics examples, three Mosteller areas and doses of 108 mg, 22.8 g and 103.2 mg, were reproduced.

The formulas were then compared on 90 adult pairs, 10 child sizes and 10 very large adults, and the spreads above are those measurements. That shows how far the formulas sit apart, not which is closest to a real person’s skin. DuBois reported his formula within 5% at most and 1.5% on average of measured areas, and Haycock a correlation of 0.998. The ranges and guideline statements come from StatPearls, KDIGO 2024, Mohamed and colleagues and the ASCO pages. Every address above was opened and checked.

Common questions

What is body surface area?

It is the area of the skin in square metres, estimated from height and weight. Adult figures usually fall between about 1.7 and 2.1 m². UK cancer patients averaged 1.71 m² for women and 1.91 m² for men in a 2010 study of 3,613, and US adults average 1.87 and 2.09 m² by Mosteller.

Which body surface area formula should I use?

Use the one your protocol names. Otherwise Mosteller is the usual default: the StatPearls cardiac index chapter uses it, and Verbraecken and colleagues recommended it in 2006 for its simplicity. Haycock was fitted from premature infants to adults, so it suits children better than DuBois, which read nearly 8% low in newborns. At healthy adult weights all five agree within about 3%.

What is a normal body surface area?

There is no single normal figure. The 1.73 m² reference dates from 1928 and is not a current average. An adult of 170 cm and 65 kg comes out at 1.75 m² by every formula to within 0.6%, and measured averages run from 1.71 to 2.09 m².

How do you calculate cardiac index?

Divide the cardiac output in litres per minute by the area in square metres. An output of 5 L/min over 1.82 m² is 2.75 L/min/m². StatPearls gives the usual range as 2.5 to 4 and says below 2 should raise suspicion of cardiogenic shock. A clinician reads it with the rest of the picture. At 175 cm and 150 kg the same 5 L/min is 1.85 by Mosteller and 1.96 by DuBois.

Why is kidney function reported per 1.73 square metres?

In 1928 McIntosh, Möller and Van Slyke put kidney measurements on a common body size, and 1.73 m² was the area derived for 25 year old North American adults. KDIGO 2024 still asks for eGFR relative to 1.73 m² in adults, and notes that an eGFR not indexed to body surface area may be indicated at extremes of body weight.

Can this calculator work out a drug dose?

It multiplies a dose per square metre by the area, which is arithmetic, and shows how far the five formulas differ. It cannot tell you what the dose should be: prescribing follows the institution protocol and the responsible clinician. For obese adults with cancer, ASCO recommends full weight based doses using actual body weight and does not support cutting a dose because of obesity alone.

Related Calculators and Reference

• BMI Calculator: the other index built from height and weight.
• BMR Calculator: energy use scaled to the same body size.
• Lean Body Mass Calculator: separates fat from the rest.
• Height Converter: feet and inches to centimetres.
• Convert Kg to Lbs: kilograms and pounds.
• eGFR Calculator: converts an indexed eGFR into mL per minute using a body surface area.
• StatPearls: Body Surface Area: a clinical review of the formulas.