The volume of 3 canine and 34 human kidneys was calculated based on a 3-dimensional ellipsoid formula. Following nephrectomy, renal volume was measured by water displacement and/or estimated by determination of renal mass. Linear regression of ultrasound volume with the water-displacement volume and renal mass was ascertained, and the coefficient of determination was calculated. Based on water-displacement measurements, the ellipsoid formula was modified empirically.