In surgery for retinal detachment, such as shortening of the eyeball, volume reducing, and indentating operations, knowledge of the volume of the subretinal fluid is important. A method based on ultrasonography was developed to measure the volume of subretinal fluid preoperatively. After establishing a topography of the fundus, the height of retinal detachment can be measured by ultrasonography at 49 defined points. The diameter of the eyeball is measured at 12 different points. From the height of retinal detachment and the diameter of the eyeball it is possible to calculate the subretinal space, which is divided into 60 segments. The total volume of subretinal fluid is calculated by adding the volumes of these 60 segments. The calculus is programmed on a computer.