Veterinary Body Surface Area (BSA) Calculator for Chemo Dosing
Calculate body surface area (m²) from body weight for chemotherapy dosing in dogs and cats. Optionally multiply by a mg/m² dose to get the total mg.
Species
Patient weight (kg)
Drug dose (mg/m²) — optional
How to use this tool
Why dose by BSA instead of mg/kg?
BSA scales more closely with metabolic rate, renal clearance, and the absolute amount of chemotherapy that can be tolerated than body weight does. A 5 kg cat doesn\'t scale linearly down to a 50 kg dog\'s dose, but their BSA ratios are closer to their tolerance ratios.
The formula
BSA (m²) = k × weight(kg)^(2/3) / 10000. The constant k is 10.1 for dogs and 10.0 for cats. The 2/3 exponent reflects the surface-to-volume scaling law.
Important caveats for small patients
BSA dosing is less accurate at the extremes. For dogs < 10 kg, the BSA-derived dose is often too high relative to drug tolerance - many oncologists use a flat mg/kg cap for small patients on protocols like doxorubicin (e.g. 1 mg/kg for dogs < 10 kg instead of the BSA dose). Always cross-check with your oncology protocol and reduce doses based on tolerance and bloodwork.
Frequently asked questions
How is BSA calculated for dogs and cats?
BSA (m²) = k × weight(kg)^(2/3) / 10000, where k = 10.1 for dogs and 10.0 for cats. The 2/3 exponent reflects the surface-to-volume scaling law.
Why dose chemotherapy by BSA instead of mg/kg?
BSA correlates more closely with metabolic rate, renal clearance and chemotherapy tolerance than body weight does. The mg/kg dose of a drug like doxorubicin would dramatically under-dose large dogs if calculated linearly.
Is BSA dosing accurate for small dogs?
Less accurate at the extremes. For dogs < 10 kg, BSA-derived doses often overshoot drug tolerance. Many oncology protocols use a flat mg/kg cap for small patients (e.g. 1 mg/kg for doxorubicin in dogs < 10 kg).