A device as well as a method for determining the diameter of a yarn balloon (B) formed by a continuous yarn at a workstation (1) of a yarn balloon forming textile machine. The workstation (1) comprises an electromagnetically functioning sensor means (33), designed and arranged in such a way that at least two interruptions of a measuring beam (42) of the sensor means (33) are caused by the yarn (5, 25) forming the yarn balloon (B) during every rotation of the yarn balloon (B) during the operation of the workstation (1), and in that the time interval between the interruptions of the measuring beam (42) can be recorded by the sensor means (33) and used for calculating the diameter of the yarn balloon (B).