A method and apparatus for determining the volume of a pouch formed in a stomach during bariatric surgery comprises a balloon catheter having an inflatable balloon located at a distal end of a catheter for locating in the stomach. A pair of spaced apart stimulating electrodes are located on the catheter within the balloon and a plurality of spaced apart sensing electrodes are located on the catheter between the stimulating electrodes. The balloon in the stomach is inflated by a saline solution to define the interior of the pouch. By applying a constant current signal to the stimulating electrodes, the volume of the pouch is determined from signals read from the sensing electrodes.