The diameter (dK) of a cylindrical section and of an end cone of a single crystal being pulled from a melt in a crucible, is determined by measuring the diameter (dK) of the single crystal at an interface with the melt while taking into account a lowering rate (vs) of a surface of the melt relative to the crucible, a lifting rate (vK) with which the crystal is raised relative to the crucible, and a conservation of mass, wherein a diameter of a cylindrical section of the single crystal, determined by means of observing a bright ring on the surface of the melt, and is used for a correction, a plausibility check or a comparison of the diameter (dK) of the single crystal.