A method for measuring a tubular strand (16, 116, 216) emerging from an extrusion device (10, 110, 210) and conveyed in a longitudinal direction, said method comprising the steps of: • in a first measurement region, guiding terahertz radiation from at least one first radiation source from the inside onto the inner surface of the tubular strand (16, 116, 216), the terahertz radiation reflected by the tubular strand (16, 116, 216) being received by at least one first radiation receiver, and at least one geometric parameter of the tubular strand (16, 116, 216) being determined from the received terahertz radiation in the first measurement region, • characterized in that in a second measurement region located downstream of the first measurement region in the conveying direction of the strand (16, 116, 216), guiding terahertz radiation from at least one second radiation source from the outside onto the outer surface of the tubular strand (16, 116, 216), terahertz radiation reflected by the tubular strand (16, 116, 216) being received by at least one second radiation receiver, and at least one geometric parameter of the tubular strand (16, 116, 216) being determined from the received terahertz radiation in the second measurement region, • a change in at least one geometric parameter of the tubular strand (16, 116, 216) between the first measurement region and the second measurement region is determined, and • at least one adjustment instruction for the extrusion device (10, 110, 210) based on a determined change in the at least one geometric parameter is determined, • the refractive index of the tubular strand (16, 116, 216) is established from the terahertz radiation received by the at least one second radiation receiver in the second measurement region, and • the refractive index established in the second measurement region is used to determine the at least one geometric parameter in the first measurement region. • in a first measurement region, guiding terahertz radiation from at least one first radiation source from the inside onto the inner surface of the tubular strand (16, 116, 216), the terahertz radiation reflected by the tubular strand (16, 116, 216) being received by at least one first radiation receiver, and at least one geometric parameter of the tubular strand (16, 116, 216) being determined from the received terahertz radiation in the first measurement region, • characterized in that in a second measurement region located downstream of the first measurement region in the conveying direction of the strand (16, 116, 216), guiding terahertz radiation from at least one second radiation source from the outside onto the outer surface of the tubular strand (16, 116, 216), terahertz radiation reflected by the tubular strand (16, 116, 216) being received by at least one second radiation receiver, and at least one geometric parameter of the tubular strand (16, 116, 216) being determined from the received terahertz radiation in the second measurement region, • a change in at least one geometric parameter of the tubular strand (16, 116, 216) between the first measurement region and the second measurement region is determined, and • at least one adjustment instruction for the extrusion device (10, 110, 210) based on a determined change in the at least one geometric parameter is determined, • the refractive index of the tubular strand (16, 116, 216) is established from the terahertz radiation received by the at least one second radiation receiver in the second measurement region, and • the refractive index established in the second measurement region is used to determine the at least one geometric parameter in the first measurement region.