A method is provided for measuring the run time of an ultrasonic pulse in the determination of the flow velocity of a gas in a breathing gas volume flow sensor (1). The breathing gas volume flow sensor (1) has a flow duct (3) with a measuring section, wherein the measuring section is arranged in parallel to or at an angle (α) smaller than 90° to the flow duct (3). The measuring section has an ultrasound transmitter (5) and an ultrasound receiver (7). The method determines the run time of the ultrasonic pulse from the receiving signal spectrum without the damping of the ultrasonic pulse having an effect on the run time measurement by an ultrasonic pulse being triggered by a transmitting pulse at the ultrasound transmitter (5). The ultrasound receiver (7) sends a receiving signal and the receiving signal is recorded as a first receiving signal data record as a function of time after the transmitting pulse. A correlation function of the first receiving signal data record is formed with the first sample data record. The run time of the ultrasonic pulse is determined from the point in time of the correlation maximum.