A method and an arrangement for ultrasonic flow measurement according to the transit-time method is provided for accurate and non-invasive flow measurement independent of the viscosity. At least one sound transducer pair is provided that has a clamp-on ultrasonic transducer operating in transmitting operation and a clamp-on ultrasonic transducer operating in receiving operation. The cross-section of a measurement tube through which a gaseous or liquid medium flows is substantially a pentagon having a base side and having sides adjacent at right angles and having sides adjacent thereto and lying opposite the base side, which include an angle of less than 180°. The sound transducer pair is arranged on the base side in order produce first sound paths. Because of flattening of the vertex lying opposite the base side, the same sound transducer pair additionally produces a second sound path.