Abstract: A method and apparatus for accurately measuring respiratory flow, while the subject breathes, without using a mouthpiece, face mask or any device about the head. The rate of change of the circumference of the rib cage (chest) and the rate of change of the circumference of the abdomen are measured using an extensible belt with series strips of piezoelectric film. The stress on the film produces an electric output proportional to the rate of application of stress when connected to a proper electronic amplifier. Calibration is performed by measuring the circumference changes while the subject performs an isovolume maneuver for several breaths or while the subject breathes through a pneumotachometer and mouthpiece at a variable rate for several breaths.