BSA Calculator

BSA Formulas Comparison

Note: H = Height (cm), W = Weight (kg). All formulas calculate BSA in square meters (m²). The Boyd equation converts weight to grams internally (Wg = kg × 1000); your input stays in kg/lb.

What is Body Surface Area (BSA)?

Body Surface Area (BSA) is the measured or calculated surface area of a human body. BSA is an important measurement in medicine, particularly for determining appropriate drug dosages, assessing metabolic mass, and calculating various physiological parameters.

Unlike body weight, BSA is a more accurate indicator of metabolic mass because surface area is less affected by abnormal adipose tissue. The average adult has a BSA of approximately 1.7 to 2.0 square meters (m²).

Clinical Uses of BSA

• Chemotherapy Dosing: Many cancer medications are dosed based on BSA to ensure appropriate therapeutic levels while minimizing toxicity
• Cardiac Index: Cardiac output is often normalized to BSA to calculate cardiac index (CI = CO/BSA)
• Renal Function: Glomerular filtration rate (GFR) is commonly indexed to BSA for standardized comparison
• Burn Assessment: The extent of burns is often calculated as a percentage of total BSA
• Medication Dosing: Various medications, especially in pediatrics, are dosed according to BSA
• Nutritional Assessment: Energy requirements and fluid needs can be estimated using BSA
• Clinical Research: BSA is used to normalize physiological measurements across different body sizes

Reference Values

• Average adult male: 1.9 m² (range: 1.7 - 2.0 m²)
• Average adult female: 1.6 m² (range: 1.5 - 1.8 m²)
• Newborn infant: approximately 0.25 m²
• 2-year-old child: approximately 0.5 m²
• 10-year-old child: approximately 1.14 m²
• 12-13 year old: approximately 1.33 m²

Important Notes

• Different formulas may give slightly different results - the Du Bois formula is most commonly used in clinical practice
• The Mosteller formula is gaining popularity due to its simplicity and ease of calculation
• For pediatric patients, the Haycock formula is often preferred
• BSA calculations assume normal body proportions and may be less accurate for individuals with extreme obesity or unusual body compositions
• Always consult with healthcare professionals for medication dosing decisions
• BSA is more reliable than body weight for drug dosing because it better correlates with many physiological functions
• The choice of formula may depend on the specific clinical application and population being studied

Frequently Asked Questions

How do I turn a mg/m² dose into a total dose?

Multiply the per-m² dose by the calculated BSA: total dose = dose/m² × BSA. For example, a 75 mg/m² regimen for a patient with a 1.85 m² BSA gives 75 × 1.85 = 138.75 mg. The dose panel above does this automatically once you enter the mg/m² value. For obese patients many centers cap BSA at 2.0 m² before multiplying to limit toxicity, so the panel offers a 2.0 m² cap toggle - but this is an active controversy and ASCO guidance often argues against routine capping. This is a reference estimate only; always defer to the exact protocol and rounding rules your treating oncologist or pharmacy uses.

Which BSA formula should I use for my situation?

Du Bois & Du Bois (1916) is the historical default and what most published nomograms reference, but the modern clinical preference is Mosteller (1987) because it's just √(height × weight / 3600) - simple enough to verify mentally and consistently within 1-2% of Du Bois across normal body sizes. For pediatric patients use Haycock (1978), which was specifically derived from infant and child data. For oncology dosing, Gehan & George (1970) is the historical standard cited in many regimens, though Mosteller is now widely accepted as equivalent. Fujimoto is appropriate for East Asian populations where Du Bois slightly overestimates BSA.

Why do the formulas give slightly different answers?

Each formula was derived from a different population: Du Bois from 9 adults in 1916, Mosteller from a 1987 regression on adults, Haycock from a 1978 study of 81 newborns to adults, Boyd from a 1935 sample of children. They are all empirical fits to bodies measured by paper-tape covering technique, which carries inherent 5-10% measurement error. Across the typical adult range the formulas agree within about 3%, but at the extremes (very small children, very large adults) the gaps widen. For routine clinical work the difference is usually under the 10% safety margin most drug dosing protocols build in.

How accurate is BSA for chemotherapy dosing?

BSA-based dosing has been the chemotherapy standard since the 1950s, but it's known to be a crude proxy for drug clearance. Multiple studies show that BSA accounts for only 15-30% of the variance in actual drug exposure - obesity, renal function, age and pharmacogenetics matter more for some drugs. Modern oncology is gradually moving toward fixed dosing, weight-tiered dosing or pharmacokinetic-guided dosing for many agents, but BSA remains the default for traditional cytotoxics (5-FU, doxorubicin, cisplatin). Capping BSA at 2.0 m² in obese patients is common to avoid overdosing, though guidelines vary.

Is BSA more useful than BMI?

They answer different questions. BMI assesses weight relative to height for health categorization (under/normal/over/obese) and is poor for muscular athletes. BSA estimates total body surface for dosing and physiology - it correlates better with metabolic rate, cardiac output and renal function than BMI does. For pharmaceutical dosing, fluid replacement and indexing physiological values (GFR/BSA, CI = CO/BSA), BSA wins. For obesity screening, cardiovascular risk and population health, BMI plus waist circumference is the practical choice. The two metrics are complementary, not competing.

How does BSA change with age in children?

A newborn has approximately 0.25 m² BSA; this doubles to ~0.5 m² by age 2, doubles again to ~1.0 m² by age 9-10, and reaches adult range (1.7-2.0 m²) around age 16-18. The ratio of BSA to body weight is much higher in infants - newborns have about 3 times more skin area per kg than adults, which is why fluid loss, hypothermia, and heat-stroke risk are disproportionately high in infants. The Haycock and Gehan & George formulas handle this curve more accurately than Du Bois at small body sizes.

Why is BSA used in burn assessment?

Burn severity is quantified as percentage of total BSA affected, which directly drives fluid resuscitation (Parkland formula: 4 mL × kg × %BSA burned over 24 hours) and triage decisions. The Rule of Nines is the field shortcut - 9% each for the head and each arm, 18% each for chest, back, and each leg, 1% for the perineum. For more precise assessment, the Lund and Browder chart adjusts these percentages by age. A burn affecting more than 20% BSA in adults (10% in children or elderly) generally requires transfer to a burn center.

Should obese patients use a capped BSA for drug dosing?

It's an active controversy. Many oncology centers cap BSA at 2.0 m² (some at 2.2 m²) for obese patients on cytotoxic chemotherapy because uncapped doses produced more toxicity without better outcomes in retrospective reviews. ASCO 2012 guidelines actually argue against routine capping for most regimens, citing better survival in obese patients dosed by actual BSA. The practical answer: cap based on the specific drug's evidence base, not a blanket rule. Always defer to the protocol your treating oncologist uses - this is an area where individual institutions and trial protocols differ.

Can BSA be measured directly instead of calculated?

Yes, but rarely. Direct BSA measurement uses 3D body scanning or the historical "paper-tape covering" technique that the original formulas were derived from. Modern 3D scanners can measure BSA to within 1% accuracy, and a few research centers use them for chemotherapy dosing in obese or unusually proportioned patients. For 99% of clinical applications, the calculated BSA from a standard formula is well within the precision needed - the inherent variability in drug pharmacokinetics is larger than the gap between calculated and measured BSA, so the extra cost of scanning is rarely justified.