A device for determining the diameter of a yarn balloon (B) formed by a running yarn at a workstation (1) of a textile machine wherein the workstation (1) is provided with a mechanical, contact scanning sensor (22), which is designed and arranged so that during the operation of the workstation (1) it is positioned by a yarn forming the thread balloon (B) in an operating position dependent on the diameter of a thread balloon (B), and a sensor device (24) is provided which detects the operating position (BS) of the scanning sensor (22).