Body Surface Area (BSA): Mosteller Formula & Medical Uses

Q: Why do chemotherapy doses use BSA?

Drug clearance and distribution scale with body size; dosing in mg/m² normalizes across patients. Example: drug at 100 mg/m², patient BSA 1.84 → 184mg per cycle. Modern oncology sometimes caps BSA at 2.0-2.2 m² for very large patients to avoid overdosing, and some drugs have moved to fixed dosing — but BSA-based dosing remains the default for most cytotoxics.

Q: What is the rule of nines for burns?

Adult body divided into 9% BSA chunks: each arm 9%, each leg 18%, front torso 18%, back torso 18%, head 9%, groin 1%. A burned arm + front torso = 27% BSA. It drives fluid resuscitation (Parkland: 4ml × kg × %BSA in first 24h). Children's heads are proportionally larger — pediatric charts adjust (Lund-Browder).

Q: What is cardiac index and how does BSA enter?

Cardiac index = cardiac output ÷ BSA (normal 2.5-4.0 L/min/m²). Raw cardiac output misleads across body sizes — 4 L/min is fine for a small adult, inadequate for a 2.2 m² athlete. Indexing by BSA makes hemodynamic values comparable. Same logic applies to GFR indexing (ml/min/1.73m²) in kidney assessment.

Q: What mistakes happen in BSA calculations?

(1) Unit errors — cm vs m, kg vs lb (pounds ÷ 2.205 first); (2) forgetting the square root; (3) dividing by 360 vs 3600; (4) using actual weight in obese patients where the protocol calls for adjusted weight; (5) rounding mid-calculation. A 10% BSA error flows directly into a 10% dose error — double-check with a second method or calculator in clinical settings.

Core answer: Body surface area (BSA) is estimated from height and weight; the standard Mosteller formula: BSA (m²) = √(height(cm) × weight(kg) / 3600). A 170 cm / 65 kg adult: √(170×65/3600) = √3.07 ≈ 1.75 m². Average adult ≈ 1.7–1.9 m². BSA drives chemotherapy dosing, burn assessment ("rule of nines"), and cardiac indexing — mg/m² dosing exists because metabolism scales with surface, not weight.

The formulas (Mosteller wins for simplicity)

Formula	Equation
Mosteller (1987)	√(h×w/3600)
Du Bois (1916)	0.007184 × h^0.725 × w^0.425
Haycock	0.024265 × h^0.3964 × w^0.5378

For 170 cm/65 kg: Mosteller 1.75, Du Bois 1.76 — differences under 2%, so Mosteller dominates clinical use (calculable on any phone).

Worked examples

Example 1 — Chemo dosing. Drug dosed at 50 mg/m²; patient 160 cm/70 kg: BSA = √(160×70/3600) = √3.11 ≈ 1.76 m² → dose = 88 mg. This is why oncology asks your height AND weight every cycle.

Example 2 — Pediatric fluids. A child 110 cm/20 kg: BSA ≈ √(110×20/3600) ≈ 0.78 m². Maintenance fluids and drug doses scale from adult values by BSA ratio (0.78/1.73 ≈ 45%).

Example 3 — Cardiac index. Cardiac output 5.0 L/min ÷ BSA 1.75 = CI 2.86 L/min/m² — the normalized number cardiologists use (normal 2.5–4.0).

The rule of nines (burns)

Adult body split in 9% blocks: head 9%, each arm 9%, front torso 18%, back torso 18%, each leg 18%, groin 1%. A burned arm + front torso = 9 + 18 = 27% TBSA — drives fluid resuscitation (Parkland: 4 mL × kg × %TBSA in 24 h) and burn-center referral thresholds (>10% in kids, >15–20% adults).

Common mistakes and myths

Dosing by weight when BSA is specified — for chemo and several pediatric drugs this misfires both ways: thin-tall patients get underdosed by weight, short-heavy overdosed.
Unit slips — Mosteller wants cm and kg; inches/pounds need conversion first (170 lb = 77 kg, not 170 kg).
Assuming BSA is measured — it's always estimated; the formulas differ by a few percent, which is acceptable clinically.
Applying adult nines to children — infants' heads are ~18% (double adults') and legs smaller; pediatric burn charts (Lund-Browder) adjust by age.
Believing BSA explains everything — renal dosing uses creatinine clearance, obesity dosing sometimes uses adjusted body weight; BSA is one scaling tool among several.

Frequently Asked Questions

How is body surface area calculated?

Mosteller formula: BSA(m²) = √(height(cm) × weight(kg) ÷ 3600). For 175cm, 70kg: √(175 × 70 ÷ 3600) = √3.403 = 1.84 m². It needs only height and weight, works across all ages, and is the most widely taught. Online calculators all implement it — the math by hand takes 30 seconds.

Which BSA formula is best?

Mosteller (1987) — recommended for simplicity and validated accuracy. DuBois & DuBois (1916, BSA = 0.007184 × H^0.725 × W^0.425) is the historic classic. Haycock, Gehan-George, and Fujimoto variants differ by <3% in typical ranges. Pediatrics often prefers Haycock. The formulas agree within 3-5% — pick your institution's standard and stay consistent.

Why do chemotherapy doses use BSA?

Drug clearance and distribution scale with body size; dosing in mg/m² normalizes across patients. Example: drug at 100 mg/m², patient BSA 1.84 → 184mg per cycle. Modern oncology sometimes caps BSA at 2.0-2.2 m² for very large patients to avoid overdosing, and some drugs have moved to fixed dosing — but BSA-based dosing remains the default for most cytotoxics.

What is the rule of nines for burns?