Body Square Meter Calculator: BSA Formula & Uses

This calculator works out your body surface area (BSA) in square meters using the Mosteller formula. You enter your height and weight, and it returns your estimated BSA, which is used in medicine to dose certain drugs and assess physiological conditions. The calculation is quick and requires only your height and weight.

Formula

The Mosteller formula is the most commonly used equation for estimating body surface area:

BSA (m²) = √(height (cm) × weight (kg) / 3600)

Where:

• height is in centimeters (cm)
• weight is in kilograms (kg)
• BSA is in square meters (m²)

The constant 3600 is derived from the original formula that used height in inches and weight in pounds; it converts those units into metric so that the result is in square meters.

Worked Example

Let’s use the default values: height = 170 cm, weight = 70 kg.

1. Multiply height by weight: 170 × 70 = 11,900
2. Divide by 3600: 11,900 / 3600 = 3.3056
3. Take the square root: √3.3056 ≈ 1.818

So the estimated BSA is 1.82 m² (rounded to two decimals). This matches the calculator output.

Why Body Surface Area Matters

BSA is not just a geometric curiosity; it correlates better with metabolic mass than body weight alone. That is why it is used for:

• Chemotherapy dosing – many anti-cancer drugs are dosed per m².
• Cardiac output calculations – the cardiac index is cardiac output divided by BSA.
• Assessing burn extent – the “rule of ninesâ€� uses BSA to estimate burn area.
• Renal function – estimated glomerular filtration rate (eGFR) is often normalized to 1.73 m².

For an average adult, BSA is roughly 1.7–2.0 m². Children have lower values, and large adults can exceed 2.5 m².

How to Measure Your Height and Weight Correctly

To get an accurate BSA, you need accurate inputs:

• Measure height without shoes, standing straight against a wall or stadiometer.
• Weigh yourself in the morning, after fasting, with minimal clothing.
• Use metric units (cm and kg) for the Mosteller formula. If you have feet/inches and pounds, convert first: 1 inch = 2.54 cm, 1 lb = 0.4536 kg.

Even a small error in height or weight will change the BSA. For instance, a 1 cm error in height changes BSA by about 0.01 m², and a 1 kg error changes it by about 0.01 m².

Other BSA Formulas

The Mosteller formula is popular because it is simple and accurate for adults. But other equations exist, each with slight differences:

• Du Bois & Du Bois (1916): BSA = 0.007184 × height^0.725 × weight^0.425 (height in cm, weight in kg)
• Haycock (1978): BSA = 0.024265 × height^0.3964 × weight^0.5378
• Gehan & George (1970): BSA = 0.0235 × height^0.42246 × weight^0.51456

These formulas give results within a few percent of each other for typical body shapes. The Mosteller formula tends to be slightly more accurate for children and obese adults than the older Du Bois formula.

Common Mistakes and Limitations

People often mix up units. The Mosteller formula requires centimeters and kilograms. If you enter pounds and inches, you’ll get a wildly wrong number. Also, the formula assumes a “normal� body composition – it does not account for muscle mass, fat distribution, or amputations. For very muscular or very obese individuals, BSA may be over- or underestimated, but it is still used in clinical practice because it is practical.

Another mistake is rounding too early. Keep full precision during the calculation and round only the final result to two decimals.

Finally, remember that BSA is an estimate, not a direct measurement. The actual surface area of the human body is difficult to measure directly; the formulas are validated against a limited sample of people.

When to Use This Calculator

Use this calculator if you need a quick BSA estimate for educational purposes or to understand medical dosing references. It is not a substitute for professional medical advice. Always consult a doctor or pharmacist for actual drug dosing.

The calculator is also useful for fitness tracking – some sports science equations use BSA to estimate metabolic rate or sweat loss.

For children, the Mosteller formula works well, but pediatric dosing often uses weight-based or age-based rules, so BSA may not be needed.

Frequently Asked Questions

What is a normal body surface area?

For an average adult, BSA is about 1.7 m² for men and 1.6 m² for women. It varies with height and weight – taller and heavier people have larger BSA. There is no “normal� range in the medical sense, but most adults fall between 1.5 and 2.2 m².

Why is BSA used for chemotherapy dosing?

Many chemotherapy drugs have a narrow therapeutic index – the difference between a toxic dose and an ineffective dose is small. BSA correlates with liver and kidney function, which clear these drugs, so dosing per m² helps reduce toxicity while maintaining efficacy.

Can I calculate BSA in imperial units?

Yes, but you must convert to metric first. The Mosteller formula can be written as BSA = √(height (in) × weight (lb) / 3131), but it is safer to convert to cm and kg and use the standard formula.

Does BSA change with age?

BSA changes as you grow and as body composition changes. It increases during childhood and adolescence, then remains relatively stable in adulthood, but may increase with weight gain or decrease with weight loss. It does not automatically change with age alone.

Is BSA the same as BMI?

No. BMI (body mass index) is weight divided by height squared (kg/m²) and is a measure of body fatness. BSA is the total surface area of the body (m²). They measure different things, though both use height and weight.

How accurate is the Mosteller formula?

For most adults, the Mosteller formula is accurate to within 5% of more complex methods. It is less accurate for very tall, very short, or severely obese individuals, but it remains the standard in many clinical settings because of its simplicity.

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in h | Height | cm | 170 | 0
in w | Weight | kg | 70 | 0
out bsa | Body surface area | m² | sqrt((h * w) / 3600) | 2