A method (600) for estimating patient airway flow in a non-invasive ventilator system. The method includes the steps of: (i) determining (630) an estimated gas flow at the proximal end of the tubing (ii) determining (640) a proximal pressure error value by subtracting the measured pressure at the proximal end of the tubing from the estimated pressure at the proximal end of the tubing (iii) compensating for the determined proximal pressure estimate error value (iv) compensating (654) for an error in the estimated gas flow at the proximal end of the tubing by feeding the estimate back into a sum of accumulated flows (v) determining (656) an estimated gas flow leak (vi) monitoring (660) for a leak in the non-invasive ventilator system (vii) determining (670) a gas flow leak factor (viii) adjusting (680) the estimated gas flow leak and (ix) compensating (690) for bias in the patient airway flow.
