Dosage by Body Surface Area (BSA)

Some medications — particularly chemotherapy agents — are dosed based on body surface area (BSA) rather than weight alone. BSA correlates more closely with metabolic rate and organ function than body weight, making it a more accurate way to individualize drug doses. BSA-based dosing is the standard in oncology and is also used in pediatrics for certain medications.

Why BSA Instead of Weight?

Two patients can weigh the same but have very different body compositions. A tall, lean patient and a short, stocky patient at 70 kg will process drugs differently because their organ sizes, blood volumes, and metabolic rates differ. BSA accounts for both height and weight, providing a better estimate of how the body will handle a drug.

Key fact: The average adult BSA is approximately $1.7 \text{ m}^2$. Values typically range from about $1.5$ to $2.2 \text{ m}^2$ for adults.

The Mosteller Formula

The most commonly used formula in clinical practice is the Mosteller formula:

$\text{BSA (m}^2\text{)} = \sqrt{\frac{\text{Height (cm)} \times \text{Weight (kg)}}{3600}}$

This formula uses height in centimeters and weight in kilograms. The constant 3600 is a simplification factor that makes the formula practical for bedside use.

Unit Conversions You May Need

Before applying the formula, convert measurements if needed:

Calculating BSA: Step by Step

Example 1: Adult Patient

Patient: Height 170 cm, Weight 80 kg.

Step 1: Multiply height by weight.

$170 \times 80 = 13{,}600$

Step 2: Divide by 3600.

$\frac{13{,}600}{3600} \approx 3.778$

Step 3: Take the square root.

$\text{BSA} = \sqrt{3.778} \approx 1.94 \text{ m}^2$

Answer: The patient’s BSA is approximately 1.94 m².

Example 2: Pediatric Patient

Patient: A child is 3 feet 4 inches tall and weighs 40 lb.

Step 1: Convert height to cm.

$3 \text{ ft } 4 \text{ in} = 40 \text{ in} \times 2.54 = 101.6 \text{ cm}$

Step 2: Convert weight to kg.

$\frac{40}{2.2} \approx 18.2 \text{ kg}$

Step 3: Apply the Mosteller formula.

$\text{BSA} = \sqrt{\frac{101.6 \times 18.2}{3600}} = \sqrt{\frac{1849.12}{3600}} \approx \sqrt{0.5136} \approx 0.72 \text{ m}^2$

Answer: The child’s BSA is approximately 0.72 m².

Using BSA to Calculate a Dose

Once you have the BSA, the dose calculation is straightforward. Drug orders will be written as a dose per square meter (e.g., mg/m²):

$\text{Dose (mg)} = \text{BSA (m}^2\text{)} \times \text{Prescribed dose (mg/m}^2\text{)}$

Example 3: Chemotherapy Dosing — Cisplatin

Order: Cisplatin 75 mg/m² IV. Patient’s BSA is 1.94 m².

$\text{Dose} = 1.94 \times 75 = 145.5 \text{ mg}$

Answer: The patient should receive 145.5 mg of Cisplatin. (In practice, oncology doses may be rounded to a practical amount per facility protocol.)

Example 4: Pediatric BSA Dosing — Methotrexate

Order: Methotrexate 3.3 mg/m² PO daily. The child’s BSA is 0.72 m².

$\text{Dose} = 0.72 \times 3.3 = 2.376 \text{ mg}$

Rounding per facility protocol:

Answer: Administer approximately 2.4 mg of Methotrexate PO daily.

Complete Workflow: From Measurements to Dose

Scenario: A patient is 5’8” tall and weighs 176 lb. The oncologist orders Carboplatin 300 mg/m² IV.

Step 1: Convert height.

$5\text{ ft } 8\text{ in} = 68 \text{ in} \times 2.54 = 172.72 \text{ cm}$

Step 2: Convert weight.

$\frac{176}{2.2} = 80 \text{ kg}$

Step 3: Calculate BSA.

$\text{BSA} = \sqrt{\frac{172.72 \times 80}{3600}} = \sqrt{\frac{13{,}817.6}{3600}} \approx \sqrt{3.838} \approx 1.96 \text{ m}^2$

Step 4: Calculate dose.

$\text{Dose} = 1.96 \times 300 = 588 \text{ mg}$

Answer: The patient should receive approximately 588 mg of Carboplatin IV.

BSA Reference Table (Approximate Values)

These values are calculated using the Mosteller formula and can be used as a quick reference to verify your calculations.

Common Mistakes to Avoid

1. Using inches and pounds directly in the Mosteller formula. The formula requires height in centimeters and weight in kilograms. Using imperial units without converting will produce a wildly incorrect BSA.
2. Forgetting the square root. The formula is $\sqrt{\frac{H \times W}{3600}}$, not $\frac{H \times W}{3600}$. Without the square root, your answer will not be in m² and will be far too large.
3. Rounding BSA too aggressively. For chemotherapy, even small errors in BSA translate to significant dose differences. Round BSA to two decimal places (the hundredths place) per standard nursing math guidelines.
4. Confusing mg/m² with mg/kg. These are completely different dosing methods. A mg/m² dose requires BSA; a mg/kg dose requires weight only. Using the wrong method could result in a dangerous dose.

Practice Problems

Test your understanding with these problems. Click to reveal each answer.

Key Takeaways

• BSA-based dosing is used when a more precise individualized dose is needed — especially in oncology and pediatrics
• The Mosteller formula is: $\text{BSA} = \sqrt{\frac{\text{Height (cm)} \times \text{Weight (kg)}}{3600}}$
• Always convert to centimeters and kilograms before using the formula
• To find the dose: multiply BSA by the prescribed mg/m² value
• Average adult BSA is approximately $1.7 \text{ m}^2$
• Some drugs have maximum dose caps regardless of BSA (e.g., Vincristine 2 mg) — always check
• Round BSA to two decimal places (the hundredths place) to maintain dose accuracy

Return to Math for Nurses for more topics in this section.