BSA Calculator

A BSA calculator estimates body surface area from height and weight, giving a single body-size value in square meters. Clinicians and researchers use body surface area because some physiologic processes scale more closely with surface area than with weight alone. This page explains the exact Mosteller equation used by the interactive calculator, shows a matching worked example, and keeps the medical context separate from individualized care.

What body surface area measures

Body surface area, abbreviated BSA, is an estimate of the body’s external surface. It is not measured with a tape in routine care; instead, it is calculated from height and weight using an equation derived from population data. The value is commonly expressed as square meters, written as m². For many adults, BSA falls near 1.6 to 2.2 m², but that observation is only descriptive. The calculator does not label a BSA as healthy or unhealthy.

BSA appears in clinical literature, chemotherapy protocols, burn assessment discussions, cardiac index calculations, and medication labeling. Those uses do not mean a person should self-adjust medication from a web result. A protocol may specify a particular equation, dose cap, organ-function adjustment, or rounding rule. If the number will be used in care, confirm the method with the clinician, pharmacist, or written protocol involved.

For broader body-size context, the BMI calculator describes weight relative to height, the healthy weight calculator gives population-based weight ranges, and the waist-to-height ratio calculator focuses on central body shape. Those tools answer different questions from BSA.

How this calculator works

The calculator accepts either metric or imperial units. If you choose metric, it reads height directly as centimeters and weight directly as kilograms. If you choose imperial, it converts inches to centimeters by multiplying by 2.54, and pounds to kilograms by multiplying by 0.453592. It then applies one equation to the converted values.

The calculation uses the Mosteller formula only. Although the page metadata mentions other historical formulas, the current interactive calculator does not calculate Du Bois, Haycock, or Boyd values. That distinction matters when you compare results across websites, hospital systems, or research articles.

Formula

The exact calculation used by the calculator is:

$\text{BSA} = \sqrt{\frac{\text{height in cm} \times \text{weight in kg}}{3600}}$

where:

• height in cm is either the metric height you entered or inches multiplied by 2.54.
• weight in kg is either the metric weight you entered or pounds multiplied by 0.453592.
• BSA is body surface area in square meters.

The displayed result is rounded to two decimals. The output also lists the formula name, the height used, and the weight used, so you can see whether a unit conversion happened before the formula was applied.

Example: calculating body surface area

Suppose the calculator is set to metric, with height 170 cm and weight 70 kg. The calculation multiplies the two inputs:

$170 \times 70 = 11900$

It divides by 3600:

$11900 \div 3600 = 3.305555\ldots$

Then it takes the square root:

$\sqrt{3.305555\ldots} = 1.818119\ldots$

Rounded as the calculator displays it, the body surface area is 1.82 m². The result panel should also show Formula: Mosteller, Height used: 170 cm, and Weight used: 70 kg.

For an imperial example, enter 67 in and 154 lb. the calculator converts 67 inches to 170.2 cm and 154 pounds to 69.9 kg before applying the same formula. The resulting BSA is again about 1.82 m², with small differences coming from conversion and rounding.

Interpreting the result

BSA is a scale factor, not a diagnosis. A larger person usually has a larger BSA, but the number does not tell you whether weight, body composition, nutrition, hydration, cardiovascular risk, or medication exposure is appropriate. It also does not account for pregnancy, edema, amputation, severe obesity, infant body proportions, or other situations where a standard height-and-weight equation may be less representative.

Different formulas can produce slightly different values. The Du Bois and Du Bois equation is historically important and has been used in many clinical settings, while Mosteller is popular because it is simple and usually close to other adult estimates. Pediatric and oncology protocols may specify exactly which method to use. When documentation matters, record the equation name along with the numeric BSA.

Common mistakes

• Mixing inches with kilograms or centimeters with pounds without conversion.
• Assuming every BSA calculator uses the same equation.
• Rounding height or weight too aggressively before calculating.
• Treating BSA as a health score instead of a body-size estimate.
• Using BSA to change a dose without a prescribing protocol.
• Comparing a rounded result with an unrounded value from another system.

Limitations and medical disclaimer

This calculator is for education only and is not medical advice. It does not diagnose, prescribe, or recommend individualized treatment. BSA-based dosing and clinical interpretation can depend on age, diagnosis, lab results, organ function, pregnancy status, body composition, and protocol-specific limits. Consult a qualified healthcare professional for diagnosis, treatment, medication dosing, or interpretation of a clinical result.

Sources

• NIH/NLM StatPearls, Body Surface Area — clinical background on body surface area formulas and uses.
• Mosteller RD, Simplified calculation of body-surface area — original Mosteller equation publication indexed in PubMed.
• Verbraecken J et al., Body surface area in normal-weight, overweight, and obese adults — comparison of BSA equations and their variation.