Body Surface Area Calculator

What is body surface area?

Body surface area (BSA) is the total area of the outside of the body, measured in square metres (m²). A typical adult has a BSA of roughly 1.6 to 2.0 m², a newborn about 0.2 m², and a very large adult can exceed 2.5 m².

Nobody measures BSA directly in routine care, because wrapping and measuring a body is impractical. Instead it is estimated from two numbers that are easy to obtain, height and weight, using a formula fitted to measured data. Weight alone says little about size, and height alone says nothing about build, but together they track surface area well.

BSA matters because many physiological quantities scale with the surface of the body rather than with its mass: heat loss, fluid needs, organ blood flow and the way some drugs are cleared. Dividing by BSA lets clinicians compare a small patient and a large one on equal terms.

How to use this calculator

1. Choose US units (feet, inches and pounds) or Metric units (centimetres or metres, and kilograms).
2. Enter your height. In US mode you can fill in feet only, inches only, or both. In metric mode pick centimetres or metres with the selector.
3. Enter your weight. The weight is read in pounds in US mode and in kilograms in metric mode, so make sure the unit system matches the number you typed.
4. Press Calculate BSA. The result is shown in both square feet and square metres, with the unit of your chosen system first.

If height is left blank or is zero, the result is zero, because the formula multiplies by a power of the height. Weight is required.

BSA formulas

Dozens of formulas have been published since the late nineteenth century. They all have the same shape, a constant times weight raised to one power times height raised to another, and they differ in the constants and exponents fitted to their original samples. This calculator uses the best known one, published by Du Bois and Du Bois in 1916.

BSA (m²) = 0.007184 × W^0.425 × H^0.725Du Bois and Du Bois. W is weight in kilograms, H is height in centimetres.

The same equation can be written with height in metres, in which case the constant becomes 0.20247. Square feet follow from the exact relation 1 m² = 10.7639 ft².

Worked example

Take a person who is 168 cm tall and weighs 62 kg. Raise the weight to the power 0.425 (62^0.425 ≈ 5.778) and the height to 0.725 (168^0.725 ≈ 41.053), then multiply by the constant.

0.007184 × 5.778 × 41.053 ≈ 1.704 m²

Mosteller's formula gives √(168 × 62 ÷ 3600) = √2.893 ≈ 1.701 m², a difference of 0.003 m². In square feet the Du Bois result is 1.704 × 10.7639 ≈ 18.34 ft².

For a US-units check, someone who is 5 ft 6 in and weighs 140 lb is 167.64 cm and 63.50 kg, which comes to about 1.7188 m², or 18.50 ft².

Typical BSA by height and weight

The grid lists Du Bois estimates in m². It is a quick way to sanity-check a result: moving 10 kg heavier at the same height adds about 0.1 m², and 10 cm taller at the same weight adds roughly 0.06 to 0.08 m².

Children are much smaller on this scale. A 1-year-old is typically a little over 0.4 m², and adult-range values of 1.5 to 1.7 m² are reached during the teenage years.

Where BSA is used

• Drug dosing. Many chemotherapy agents are prescribed in milligrams per square metre. A 1.7 m² patient prescribed 100 mg/m² receives 170 mg.
• Cardiac index. Cardiac output (litres per minute) divided by BSA gives the cardiac index, which allows comparison between patients of different size. An output of 5.4 L/min in a person with a BSA of 1.8 m² is an index of 3.0 L/min/m².
• Kidney function. Glomerular filtration rate is conventionally reported per 1.73 m². A measured GFR of 90 mL/min in someone whose BSA is 1.9 m² corresponds to 90 × 1.73 ÷ 1.9 ≈ 82 mL/min/1.73 m².
• Burns and skin conditions. The share of the body surface affected guides fluid replacement and treatment. Tools such as the rule of nines work in percentages of total BSA.
• Energy and heat. Basal heat production correlates with surface area, which is one reason size features in BMR, RMR and TDEE estimates.

This calculator gives an estimate for general information and education. Clinical dosing must follow a prescriber's protocol, which may specify a particular formula, a capped BSA or a different weight measure. It is not medical advice; see the disclaimer.

Limitations

Every BSA formula is a statistical fit, not a measurement. Accuracy is lowest at the extremes of body size, in people with a large amount of oedema or very high or very low body fat, and in infants. Two formulas can disagree by several percent for the same patient.

Height and weight must be current. Using a self-reported height or an old weight is the most common source of error, and mixing up pounds and kilograms changes the answer by a factor of about 2.2^0.425, or roughly 40 percent. For a broader picture of body size see the BMI calculator and the ideal weight calculator.