Method for determining a dimension (D, H) of a tool with cutting edges by means of a device comprising a first light source, an image sensor and an evaluation unit, wherein light is emitted parallel to a first axis by the light source and a second axis is assignable to the image sensor, which axis runs orthogonally to the image sensor, wherein the first axis and the second axis are arranged inclined relative to each other, wherein further the light emitted by the first light source is reflected by the cutting edge of the tool in such a way that the reflected light generates light points arranged in a row on the image sensor, wherein positions (xL, zL) of the light points are determined by the evaluation unit and the dimension (D, H) of the tool is determined based on the positions (xL, zL) of the light points, characterized in that - before determining the dimension (D, H) of the tool, a reference body is connected to a tool holder of the processing machine and measured using the image sensor, - during the subsequent determination of the dimension (D, H) of the tool, the tool is connected to the tool holder of the processing machine and a correction value (Qx, Qy, Qz) is formed based on the results of the measurement of the reference body, with the correction value containing the information about the exact location of the tool holder relative to reference coordinates (Vmx, Vmy, Vmz) of the image sensor, - the dimension (D, H) of the tool is determined based on the positions (xL, zL) of the light points, taking into account the correction value (Qx, Qy, Qz). light is emitted parallel to a first axis by the light source and a second axis is assignable to the image sensor, which axis runs orthogonally to the image sensor, wherein the first axis and the second axis are arranged inclined relative to each other, wherein further the light emitted by the first light source is reflected by the cutting edge of the tool in such a way that the reflected light generates light points arranged in a row on the image sensor, wherein positions (xL, zL) of the light points are determined by the evaluation unit and the dimension (D, H) of the tool is determined based on the positions (xL, zL) of the light points, characterized in that - before determining the dimension (D, H) of the tool, a reference body is connected to a tool holder of the processing machine and measured using the image sensor, - during the subsequent determination of the dimension (D, H) of the tool, the tool is connected to the tool holder of the processing machine and a correction value (Qx, Qy, Qz) is formed based on the results of the measurement of the reference body, with the correction value containing the information about the exact location of the tool holder relative to reference coordinates (Vmx, Vmy, Vmz) of the image sensor, - the dimension (D, H) of the tool is determined based on the positions (xL, zL) of the light points, taking into account the correction value (Qx, Qy, Qz). - before determining the dimension (D, H) of the tool, a reference body is connected to a tool holder of the processing machine and measured using the image sensor, - during the subsequent determination of the dimension (D, H) of the tool, the tool is connected to the tool holder of the processing machine and a correction value (Qx, Qy, Qz) is formed based on the results of the measurement of the reference body, with the correction value containing the information about the exact location of the tool holder relative to reference coordinates (Vmx, Vmy, Vmz) of the image sensor, - the dimension (D, H) of the tool is determined based on the positions (xL, zL) of the light points, taking into account the correction value (Qx, Qy, Qz).