The invention relates to a method for verifying a calibration of an ultrasonic spirometer, the method comprising the following steps: a) determining an actual value of a distance between a first ultrasonic transducer and a second ultrasonic transducer of a spirometer； b) determining a difference between the actual value of the distance and a nominal value of the distance that is assigned to the spirometer； and c1) accepting an actual calibration of the spirometer if an absolute value of the difference is smaller than or equal to a first threshold value, or c2) refusing an actual calibration of the spirometer if an absolute value of the difference is bigger than the first threshold value, wherein the first threshold value is 5 % of the nominal value of the distance. The invention further relates to a spirometer that is adapted to carry out this method as well as to a method for calibrating a spirometer.
