In physiology and medicine, the body surface area (BSA) is the measured or calculated surface of a human body. BSA is a measurement used in many medical tasks. 
Various Body Surface Area formulas have been developed over the years, originally by Dr. Du Bois, followed by Gehan and George, Haycock, Boyd and Mosteller. These formulas all give slightly different results. The Mosteller formula is mostly recommended because it is simple and is easily memorized and calculated with a hand-held calculator.

The formula used is:
BSA (m²) = ([Height (cm) x Weight (kg)]/ 3600) ^½   

The Mosteller formula was adopted for use by the Pharmacy and Therapeutics Committee of the Cross Cancer Institute, Edmonton, Alberta, Canada.

References :

Mosteller RD. Simplified calculation of body surface area. N Engl J Med. 1987 Oct 22;317(17):1098.