Medical Equations

Body Surface Area (BSA) Calculator

Body Surface Area formula based on height and weight

Solution

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Worked Examples

Adult Oncology

Mosteller example for a 170 cm, 70 kg adult

This is the common bedside workflow for adult chemotherapy dosing when height and weight are already charted in metric units.

  1. Enter 170 for height and keep the unit set to cm.
  2. Enter 70 for weight and keep the unit set to kg.
  3. Choose the Mosteller formula.
  4. Read the BSA result of about 1.818 m².
  5. Use that BSA value to calculate the medication dose ordered in mg/m².

Mosteller is often favored because it is easy to verify manually and is accepted in many adult dosing protocols.

Imperial Inputs

Compare formulas for a 69 inch, 176 lb adult

This example shows how the calculator handles imperial inputs while still applying the metric-based BSA equations behind the scenes.

  1. Enter 69 for height and switch the unit to inches.
  2. Enter 176 for weight and switch the unit to pounds.
  3. Select the DuBois formula first and note the result.
  4. Switch to Mosteller or Haycock to compare how much the estimate changes.
  5. Use the selected institution-approved value for the downstream clinical calculation.

The formula comparison is especially useful when the chart or protocol names a specific BSA method.

Pediatric Dosing

Haycock example for a 110 cm, 18 kg child

Haycock is commonly discussed in pediatric settings because it performs well across smaller body sizes.

  1. Enter 110 cm for height and 18 kg for weight.
  2. Select the Haycock formula.
  3. Read the resulting BSA of about 0.78 m².
  4. Compare it with Mosteller if you want to see how close the methods are.
  5. Document the BSA method used so the dosing workflow stays consistent.

Pediatric oncology and subspecialty protocols may specify the exact BSA method to use for dose calculations.

Mosteller Formula

The simplest and most widely used BSA formula. Takes the square root of the product of height and weight divided by 3600. Preferred in most clinical settings for adults and children.

BSA = √(Height_cm × Weight_kg / 3600)

DuBois & DuBois Formula

The original BSA formula from 1916, derived from measurements of nine individuals. Widely cited and validated but may underestimate BSA in obese patients.

BSA = 0.007184 × H^0.725 × W^0.425

Haycock Formula

Developed with pediatric patients in mind, this formula performs well across a wide range of body sizes from neonates to adults. Often preferred for chemotherapy dosing in children.

BSA = 0.024265 × H^0.3964 × W^0.5378

How It Works

Body Surface Area (BSA) estimates the total area of the external surface of the human body, measured in square meters (m²). Unlike body weight alone, BSA accounts for both height and weight, providing a more physiologically relevant measure for calculating drug dosages, estimating metabolic rate, indexing cardiac output, and assessing burn severity. The average adult BSA is approximately 1.7 m².

Example Problem

An oncologist needs to calculate BSA for a patient who is 170 cm tall and weighs 70 kg.

  1. Record the patient's height as 170 cm and weight as 70 kg, which are already in the units required by the Mosteller equation.
  2. Multiply height by weight: 170 × 70 = 11,900.
  3. Divide by 3600 to normalize the product: 11,900 / 3600 = 3.3056.
  4. Take the square root of 3.3056 to get the body surface area: BSA = 1.818 m².
  5. If the chemotherapy protocol calls for 75 mg/m², multiply 75 × 1.818 to estimate a dose of about 136.4 mg.

Many order sets round the final dose according to institution policy, so the calculated BSA is the starting point rather than the final administration instruction.

Formula Guide

These variables appear in most BSA equations. The calculator converts imperial inputs to metric before applying the selected formula.

H = Height (cm)

Patient height measured in centimeters. If you enter inches, the calculator converts them to centimeters first.

W = Weight (kg)

Patient weight measured in kilograms. If you enter pounds, the calculator converts them to kilograms first.

BSA = Body Surface Area (m²)

Estimated external body surface used for dosing, indexing cardiac output, and other clinical normalization tasks.

Key Concepts

BSA is critical in oncology for chemotherapy dose calculation, where many protocols express doses in mg/m². In cardiology, BSA is used to calculate Cardiac Index (CI = CO / BSA), which normalizes cardiac output to body size. Burn assessment uses BSA to estimate total body surface area affected, guiding fluid resuscitation via the Parkland formula.

Applications

  • Chemotherapy dose calculation (mg/m² dosing)
  • Cardiac Index normalization (CI = CO / BSA)
  • Burn severity assessment and fluid resuscitation
  • Renal function indexing (GFR normalized to BSA)
  • Pediatric drug dosing across varied body sizes

Common Mistakes

  • Using the wrong height/weight units (formulas require cm and kg)
  • Applying the DuBois formula to obese patients where it may underestimate BSA
  • Not verifying which BSA formula your institution requires
  • Confusing BSA (m²) with BMI (kg/m²) — they measure different things

Frequently Asked Questions

Which BSA formula should I use?

For most adult clinical applications, the Mosteller formula is recommended due to its simplicity and accuracy. For pediatric patients, the Haycock formula may be more appropriate. The DuBois formula is historically significant but may underestimate BSA in obese individuals. Always follow your institution's protocol for the preferred formula.

Why is BSA used instead of body weight for drug dosing?

BSA correlates more closely with metabolic rate and organ size than body weight alone. A tall, lean patient and a short, heavy patient with the same weight have different metabolic capacities. BSA-based dosing helps standardize drug exposure across diverse body compositions, particularly for drugs with narrow therapeutic windows like chemotherapy agents.

What is a normal BSA for an adult?

Average adult BSA ranges from approximately 1.6 to 1.9 m². For adult males, the average is about 1.9 m², while for adult females it is about 1.6 m². Neonates typically have a BSA of 0.2 to 0.25 m², and children's values fall between neonatal and adult ranges depending on age and size.

Is BSA the same as BMI?

No. BSA estimates the total external body surface area in square meters, while BMI estimates body mass relative to height in kg/m². BMI is commonly used for weight classification, but BSA is more often used for drug dosing and physiologic indexing.

How different are the Mosteller, DuBois, and Haycock formulas?

For many average-sized adults, the formulas produce very similar values. Differences become more noticeable at the extremes of body size, in pediatrics, or in obesity. That is why institutional policy matters more than chasing tiny differences between formulas.

Can I enter height in inches and weight in pounds?

Yes. The calculator accepts imperial units and converts them to centimeters and kilograms before applying the selected equation, so you can work from charted bedside values without doing the conversions yourself.

Reference: Mosteller RD. Simplified calculation of body-surface area. N Engl J Med. 1987;317(17):1098.

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