Sonographic images of the gallbladder enable satisfactory approximation of gallbladder volume using the sum-of-cylinders method. The sum-of-cylinder measurements, however, are moderately cumbersome and time consuming to perform. In this investigation, in vitro and in vivo testing was done to determine that a simple ellipsoid method applied to sonographic gallbladder images yields reasonable volume approximations that are comparable to the volumes calculated by the sum-of-cylinders method. Findings from a water-bath experiment showed that measurement of gallbladder volume by the ellipsoid method closely approximated the true volume with a mean difference of about 1.0 ml. The results of in vivo studies in five volunteers demonstrated that the gallbladder contracted substantially after a fatty meal and that volumes calculated by the ellipsoid and sum-of-cylinders methods were nearly identical. Thus, a simple ellipsoid method, requiring negligible time, may be used to approximate satisfactory gallbladder volume for clinical or investigative studies.