Body Surface Area Calculator

What body surface area is and where it is used

Body surface area (BSA) is the total area of your skin, in square meters. Measuring it directly is slow — early researchers coated or wrapped bodies and measured the pieces — so in practice it is estimated from height and weight with a formula. A typical adult is somewhere around 1.6 to 2.0 m².

BSA is used in medicine as a measure of body size that grows more slowly than weight. Common uses include:

• Dosing some medicines, notably many cancer drugs, which are prescribed in milligrams per square meter.
• Cardiac index: cardiac output divided by BSA, so heart pumping can be compared between people of different sizes.
• Indexing other measurements, such as kidney filtration rates reported per 1.73 m², and heart chamber sizes on echocardiograms.

Enter height and weight in either unit system. The result is the Mosteller estimate, with the other four formulas listed underneath so you can see how much they differ for your measurements. If you want a weight-for- height screening number instead, use the BMI calculator; for the non-fat part of body weight, see the lean body mass calculator.

The five BSA formulas

All five take weight (W) in kilograms and height (H) in centimeters, except Boyd, which works with weight in grams. The results are in square meters.

H

height in centimeters

W

weight in kilograms

G

weight in grams (W × 1000)

Du Bois and Du Bois fitted their formula to measurements of only nine people, and it remained the standard for decades. Mosteller published a simplified version in 1987 that needs only a square root, which is why it is so widely used. Haycock and colleagues derived theirs from a sample running from newborns to adults, and it is often preferred for children. Gehan and George refitted the power law to a larger set of earlier measurements; Boyd's weight exponent shrinks slightly as weight rises.

In US units, the calculator first converts height (1 in = 2.54 cm exactly) and weight (1 lb = 0.45359237 kg exactly), so both unit systems give the same result.

Worked example

An adult 175 cm tall who weighs 70 kg:

1. Mosteller: 175 × 70 = 12,250; 12,250 ÷ 3600 = 3.4028; √3.4028 = 1.8447, shown as 1.84 m²
2. Du Bois: 0.007184 × 70^0.425 (6.0837) × 175^0.725 (42.2866) = 1.8481, shown as 1.85 m²
3. Haycock: 0.024265 × 9.8241 × 7.7471 = 1.8468, shown as 1.85 m²
4. Gehan & George: 0.0235 × 8.9005 × 8.8633 = 1.8539, shown as 1.85 m²
5. Boyd: the exponent is 0.7285 − 0.0188 × log₁₀ 70,000 = 0.6374; 0.0003207 × 175^0.3 (4.7088) × 70,000^0.6374 (1,225.5) = 1.8507, shown as 1.85 m²

The five results span only 0.01 m², about 0.5% of the Mosteller value. That agreement does not hold at every body size. For someone 180 cm tall weighing 150 kg, the same formulas give 2.74 (Mosteller), 2.61 (Du Bois), 2.81 (Haycock), 2.78 (Gehan & George) and 2.82 m² (Boyd) — a spread of 0.21 m², nearly 8%.

Why the formulas disagree, and other limits

• Each formula reflects its sample. They were fitted to small groups, measured with different methods, decades apart. A review of 25 published BSA formulas found that for the same adult they can differ by several tenths of a square meter.
• Obesity widens the gap. In a study of adults with cancer, the five formulas here agreed closely overall, but for people with a BMI over 30, Du Bois and Mosteller gave lower values than the other three — the same pattern as the 150 kg example above.
• Body shape is ignored. Two people with the same height and weight can have different real surface areas, and none of the formulas knows about amputation, swelling or pregnancy.
• Children and infants are where formula choice matters most, because small absolute differences are a large share of a small body. Pediatric teams use the formula their protocol names.

For energy needs rather than body size, see the BMR calculator; for a weight target by height, the ideal weight calculator.