A method for determining the respiratory flow of a patient receiving bi-level respiratory therapy is provided. The method may include measuring the total air flow through a breathing area at a time approximately when the patient has completed exhalation, measuring the pressure in the breathing area, determining a constant representing a corrective flow factor by dividing the measured total air flow by the square root of the measured pressure, and determining the patient's respiratory flow by subtracting from a subsequent total air flow the product of constant and the square root of a subsequent pressure.