The invention relates to a device for measuring the diameter and/or the wall thickness of a strand, which is substantially circular in cross-section and which is guided through the device in the direction of the longitudinal axis of the strand by means of guiding means, comprising at least one transmitter for emitting terahertz radiation, wherein at least one radiation optical unit is provided, which conducts terahertz radiation emitted by the transmitter to a strand guided through the device, wherein a reflector for the terahertz radiation is arranged opposite at least one transmitter and isarranged behind the strand in the radiation direction of the terahertz radiation emitted by the at least one transmitter, and also comprising at least one receiver for receiving the tetahertz radiation emitted by the at least one transmitter and reflected on the strand and/or on the reflector, and comprising an evaluating apparatus, which is designed to determine the diameter and/or the wall thickness of the strand on the basis of the measurement signals received by the at least one receiver. The invention further relates to a corresponding method.