Body Surface Area Calculator: Formula & Uses

A body surface area (BSA) calculator estimates the total surface area of your body in square meters (m²) or square feet (ft²) from your height and weight. You enter your height and weight, and the calculator returns your estimated BSA, which is used in medicine to dose certain drugs and assess physiological parameters. The most common formula is the Du Bois formula, and this calculator uses it by default.

Formula

The Du Bois formula (1916) is the classic equation for estimating BSA:

BSA (m²) = 0.007184 × W^0.425 × H^0.725

• W = weight in kilograms (kg)
• H = height in centimeters (cm)
• BSA = body surface area in square meters (m²)

To convert to square feet, multiply m² by 10.7639 (since 1 m² = 10.7639 ft²).

Worked Example

Using the default values: height = 170 cm, weight = 70 kg.

1. Compute W^0.425: 70^0.425 ≈ 6.075
2. Compute H^0.725: 170^0.725 ≈ 41.07
3. Multiply: 0.007184 × 6.075 × 41.07 ≈ 1.79

The result is approximately 1.79 m². In square feet: 1.79 × 10.7639 ≈ 19.27 ft².

Why BSA Matters

BSA is used in medicine because many physiological processes scale with surface area rather than weight alone. For example:

• Chemotherapy dosing – Many anticancer drugs are dosed per m² of BSA to reduce toxicity and improve efficacy.
• Cardiac output – The cardiac index (cardiac output divided by BSA) normalizes heart performance across body sizes.
• Renal function – Glomerular filtration rate (GFR) is often reported per 1.73 m² of BSA.
• Burn assessment – The Rule of Nines estimates burn extent, but BSA helps calculate fluid resuscitation needs.

BSA is also used in some drug dosing for antibiotics, antifungals, and certain anesthetics. It is a more individualized measure than weight, especially for obese or very lean patients.

How to Choose the Right Formula

Several BSA formulas exist, and they give slightly different results. The most common are:

• Du Bois & Du Bois (1916) – The original formula, widely used and the default in many clinical settings.
• Mosteller (1987) – Simpler: BSA (m²) = sqrt( (height cm × weight kg) / 3600 ). Often used because it is easier to calculate mentally.
• Haycock (1978) – More accurate for children and infants.
• Gehan & George (1970) – Based on a larger dataset, sometimes used in research.

For adults, Du Bois and Mosteller give nearly identical results (within 1–2%). For children, Haycock or Mosteller may be preferred. If you are using BSA for medical dosing, always follow the formula specified by the drug protocol or your institution. This calculator uses Du Bois, which is the most widely referenced.

Common Mistakes and Limitations

BSA calculators are simple, but errors can occur:

• Unit mix-ups – Height must be in centimeters and weight in kilograms for the Du Bois formula. If you enter pounds or feet, the result will be wrong. This calculator expects cm and kg.
• Rounding too early – Rounding intermediate values can shift the result by a few hundredths of a m². Use full precision until the final step.
• Not accounting for body shape – BSA formulas are estimates based on population averages. They do not account for muscle mass, fat distribution, or amputations. For a person with unusual proportions, the estimate may be off.
• Clinical use – BSA is a derived number, not a direct measurement. For critical dosing, clinicians may adjust based on other factors like renal function, drug clearance, and toxicity.

Also, BSA is not the same as body mass index (BMI). BMI uses only height and weight to estimate body fat, while BSA estimates the total surface area. Both are used in medicine but for different purposes.

How to Read Your BSA Result

The average adult BSA is about 1.7 m² (range roughly 1.5–2.0 m²). Larger individuals will have higher BSA, smaller individuals lower. In clinical settings, a BSA of 1.73 m² is often used as the standard reference for normalizing GFR and cardiac output.

For chemotherapy, a typical dose might be 75 mg per m². So a person with BSA 1.8 m² would receive 75 × 1.8 = 135 mg. This is why accurate BSA is critical – a 10% error in BSA can lead to a 10% error in drug dose.

If your BSA seems high or low, remember it is a calculation based on height and weight. If you are very tall and lean, your BSA may be higher than average even if your weight is normal.

Alternatives to BSA

While BSA is common, some dosing strategies use other measures:

• Ideal body weight (IBW) – Used for drugs that distribute poorly into fat.
• Lean body mass (LBM) – Estimated from height, weight, and sex; better for some drugs.
• Fixed dosing – Some drugs are given at a flat dose regardless of size.
• Area under the curve (AUC) dosing – For some chemotherapies, doses are adjusted to achieve a target drug exposure.

BSA remains the most widely used for chemotherapy and many other medications because it is simple and has been validated in clinical trials.

Frequently Asked Questions

What is the difference between BSA and BMI?

BMI (body mass index) is weight divided by height squared (kg/m²) and is used to classify underweight, normal, overweight, and obesity. BSA is the total surface area of the body and is used for dosing and physiological normalization. They are different metrics; a person can have a normal BMI but a high BSA if they are tall.

Which formula is most accurate?

No formula is perfectly accurate for every individual. Du Bois and Mosteller are both widely accepted for adults. Haycock is often better for children. The differences between formulas are usually small (less than 5%) for most adults. For clinical decisions, use the formula recommended by the drug label or your clinician.

Can I use this calculator for children?

Yes, but the Du Bois formula was derived from adult data. For children, especially infants, the Haycock formula is more accurate. If you are using BSA for pediatric dosing, consult a pediatric-specific formula or your pediatrician.

Why does my BSA change if I gain weight?

BSA increases with weight because more mass means more surface area. The relationship is not linear – a 10% weight gain increases BSA by about 4% (since weight exponent is 0.425). That is why doctors recalculate BSA frequently for patients on chemotherapy.

Is BSA used for all drug dosing?

No, only for certain drugs – primarily chemotherapy, some antibiotics, and a few others. Most medications are dosed by weight or fixed dose. BSA is used when drug distribution is related to body surface area, which is the case for many cytotoxic agents.

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in h | Height | cm | 170 | 0
in w | Weight | kg | 70 | 0
out bsa | Body Surface Area | m² | 0.007184 * (w^0.425) * (h^0.725) | 2
out bsa_ft | Body Surface Area | ft² | (0.007184 * (w^0.425) * (h^0.725)) * 10.7639 | 2