In physiology and medicine, the body surface area BSA is the measured or calculated surface area of a human body. For many clinical purposes body surface area BSA is a better indicator of metabolic mass than body weight because it is less affected by abnormal adipose mass.

Various calculations have been published to arrive at the body surface area BSA without direct measurement.

In this app are two formulas:
- the most widely used is the Dubois formula, which has been shown to be equally as effective in estimating body fat in obese and non-obese patients, something the Body mass index fails to do.
A commonly used and simple one is the Mosteller formula.