In order to be able to measure a diameter in a turning machine without rotating a workpiece and thus prevent circularity and concentricity errors from entering into the diameter determination, it is proposed according to the invention to use a measuring device that is movable in both transversal directions relative to the rotation axis, thus in the X-direction and also in the Y-direction, wherein the measuring device can scan the workpiece from both sides in one clamping step without rotating the workpiece.