A method is provided for determining the diameter of a rotor of a turbomachine, rotor being equipped with rotor blades. The method involves setting the rotor with the rotor blade ring in rotation, arranging a clearance measuring device assigned to the rotor blade ring outside the region of the latter, measuring the distance to the rotor blades of the rotor blade ring which are rotating past the clearance measuring device, and using the measured distance for determining the diameter of the rotor. During measuring, the rotational speed is identical to, almost identical to or higher than the setpoint rotational speed of the rotor.