A device for determining the required length of a middle ear prosthesis, having a disk-shaped base part to which prosthesis mockups or prostheses of different lengths are fastened, which, after being detached from the base part, can be inserted by an applicator into the middle ear of a patient during an operation for length determination purposes, in which the base part is provided with accessories for measuring and/or shaping the middle ear prosthesis to be inserted, so that this provides the operator with an even more simplified handling of prosthesis mockups or prostheses during the operation; no additional separate parts have to be used and instead, the handling means are compactly provided on or integrated into the device itself.