Body Surface Area (BSA) Calculator

About the Body Surface Area (BSA) Calculator

The Body Surface Area (BSA) Calculator estimates the total surface area of the human body using six widely cited formulas. BSA is a common reference measurement in clinical medicine, used in contexts such as drug dosing, renal function normalization, cardiac index derivation, and burn assessment.

Unlike body mass index which uses only weight and height in a simple ratio, BSA formulas use exponential relationships calibrated against body-surface measurements. The Du Bois & Du Bois formula (1916) remains a widely used adult reference formula, while the Haycock formula (1978) is often used in pediatric settings. The Mosteller formula (1987) offers a simplified calculation that can be done by hand.

This calculator computes BSA using all six formulas simultaneously so you can compare results and select the most appropriate one for your clinical context. Formula selection matters — in obese patients, different formulas can diverge by up to 10%, which may significantly affect chemotherapy dosing. The calculator also provides normal BSA ranges by age and sex, percent of reference BSA, and an informational chemotherapy dosing table for context.

When This Page Helps

BSA is a critical measurement used daily in hospitals, oncology clinics, nephrology, and burn units. It compares six validated formulas side by side so you can see how each method shifts the estimate. Understanding how formulas differ is essential for clinical decision-making, especially in chemotherapy where a 5% BSA difference can mean a significant change in drug dose.

The calculator also helps medical students and healthcare professionals quickly reference normal ranges and understand which formula is most appropriate for different patient populations (adults, children, obese patients, or specific ethnic groups).

How to Use the Inputs

1. Select imperial (lbs, inches) or metric (kg, cm) units.
2. Enter your body weight.
3. Enter your height.
4. Select a primary formula from the dropdown (Du Bois is the default and most common).
5. Click a preset to quickly see results for typical body sizes.
6. Review the primary BSA result and open the comparison table to see all six formulas.
7. Refer to the normal ranges and chemotherapy dosing tables for clinical context.

Example Calculation

Tips & Best Practices

• Use the Mosteller formula for quick mental estimation: √(weight_kg × height_cm ÷ 3600).
• For chemotherapy dosing, always verify BSA with your institution's preferred formula — don't assume Du Bois.
• In obese patients (BMI > 35), discuss BSA capping with the prescribing oncologist.
• Haycock is most appropriate for patients under 18 years old.
• BSA changes with weight — recalculate before each chemotherapy cycle if weight has changed.
• The 1.73 m² reference is used to normalize GFR; abnormal BSA patients may need dose adjustments.

History of Body Surface Area Measurement

The concept of using body surface area in medicine dates to the early 1900s. In 1916, D. Du Bois and E.F. Du Bois published their landmark paper deriving a formula from direct geometric measurements of nine cadavers coated with tissue paper. Despite the small sample size, their formula has proven remarkably robust and remains widely used more than 100 years later.

The Mosteller formula (1987) was developed specifically to simplify BSA calculation. By using a square root function, it can be estimated with a simple calculator or even mental math. The Haycock formula (1978) addressed the need for pediatric-specific calculations, as children's body proportions (relatively larger head, shorter limbs) differ significantly from adults'.

Modern 3D body scanning has confirmed that the Du Bois formula is accurate to within 5% for most adults, though it systematically underestimates BSA in very tall, thin individuals and overestimates in obese subjects.

BSA in Clinical Practice

**Oncology**: BSA-based chemotherapy dosing has been standard since the 1950s. The underlying assumption is that BSA correlates with drug clearance rates, so dosing by BSA achieves more consistent plasma drug levels across patients of different sizes. However, this assumption is increasingly questioned — pharmacogenomic and pharmacokinetic individualization is seen as the future of drug dosing.

**Nephrology**: Glomerular filtration rate (GFR) is normalized to 1.73 m² BSA. When a lab reports an eGFR, it's already adjusted to this reference. For patients with very large or very small BSA, unadjusted (absolute) GFR may be more clinically relevant.

**Cardiology**: Cardiac index (CI) equals cardiac output divided by BSA, normalizing for body size. Normal CI is 2.5–4.0 L/min/m². This allows comparison of cardiac function across patients of different body sizes.

**Burn Medicine**: The Parkland formula uses BSA percentage burned to calculate fluid resuscitation volumes in the critical first 24 hours. Accurate BSA estimation is literally life-saving in major burn management.

Frequently Asked Questions

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

  BSA correlates better with physiological parameters like metabolic rate, blood volume, renal clearance, and cardiac output than weight alone. A 100 kg muscular person and a 100 kg obese person have very different physiological profiles, but their BSA values partially account for this. BSA-based dosing for cytotoxic drugs (chemotherapy) has been standard since the 1950s.

• Which BSA formula should I use?

  For adults: Du Bois is the most widely accepted clinical standard. Mosteller is nearly identical and easier to calculate by hand. For children and infants: Haycock is preferred as it was specifically validated in pediatric populations. For East Asian patients: Fujimoto may be more appropriate. In obese patients: all formulas tend to overestimate, and some oncologists cap BSA at 2.0 m².

• How much do the different formulas differ?

  For normal-weight adults, most formulas agree within 2–3%. However, at extreme weights (very low or obese), differences can reach 5–10%. The Mosteller and Du Bois formulas track very closely across all body sizes. The Boyd formula tends to produce slightly higher values in obese patients.

• What is the "1.73 m² reference" BSA?

  In 1927, researchers established 1.73 m² as the BSA of the "standard man" (25-year-old male, 70 kg, 170 cm). This value is still used as a normalization reference — for example, glomerular filtration rate (GFR) is reported per 1.73 m² BSA. It's somewhat outdated as average body size has increased.

• Is BSA accurate for obese patients?

  BSA formulas were developed using normal-weight individuals and may overestimate surface area in obese patients because fat tissue contributes less to surface area per unit mass. Some clinicians use ideal body weight or adjusted body weight for drug dosing in obese patients. There is no universal guideline — the decision depends on the specific drug and clinical scenario.

• How is BSA used in burn assessment?

  The "Rule of Nines" uses BSA to estimate burn extent (e.g., each arm = 9% BSA, each leg = 18%). The Parkland formula uses BSA burn percentage to calculate fluid resuscitation volume: 4 mL × body weight (kg) × % BSA burned for the first 24 hours.