Body Surface Area Calculator

Body Surface Area Guide

Why BSA Is Used Instead of Weight

Many physiological processes scale with BSA rather than body weight — including cardiac output, glomerular filtration rate, and metabolic rate. In drug dosing, particularly for chemotherapy and immunosuppressants, BSA-based dosing is used to account for the fact that drug distribution, metabolism, and clearance depend on body surface rather than mass alone. A 50kg and 100kg person have very different surface areas, which affects how drugs should be dosed for optimal effect without toxicity.

Formula Comparison

Mosteller (1987): BSA = √(height cm × weight kg / 3600). Simplest and widely used in clinical practice. DuBois & DuBois (1916): BSA = 0.007184 × height^0.725 × weight^0.425. The original BSA formula, derived from nine subjects. Haycock (1978): BSA = 0.024265 × height^0.3964 × weight^0.5378. Most accurate for children and neonates. Boyd: uses weight only, suitable when height is unknown. For most clinical purposes, the differences between formulas are small (under 5%) and do not affect treatment

Average BSA Values

Average adult male: approximately 1.9 m². Average adult female: approximately 1.6 m². Average 10-year-old: approximately 1.14 m². Average newborn: approximately 0.25 m². The original BSA-based drug dosing assumed a standard 1.73 m² reference value — this is still used as the normalisation factor in eGFR (estimated glomerular filtration rate) calculations on kidney function blood tests.

Burn Surface Area Estimation

In burn injury assessment, the Rule of Nines estimates which percentage of BSA is affected: head and neck 9%, each arm 9%, each leg 18% (front 9%, back 9%), anterior trunk 18%, posterior trunk 18%, perineum 1%. For children, the Lund and Browder chart adjusts these percentages because children have proportionally larger heads. Burns covering more than 15% BSA in adults (10% in children) are considered major burns requiring specialist burns unit care.