A system and method of calculating an accurate estimate of pulmonary mechanics of a patient, including but not limited to compliance, resistance, and plateau pressure without modification of ventilator flow pattern. The accurate estimation of pulmonary mechanics is derived from airway pressure and flow sensors attached to the patient using novel mathematical models. These estimated figures for pulmonary mechanics (respiratory system compliance and resistance) are important for monitoring patient treatment efficacy during mechanical ventilation and ensuring alveoli do not over distend to avoid baro- and/or volutrauma, especially in patients with restrictive lung diseases. The subject method of calculating these accurate estimated figures for pulmonary mechanics is based on linear or non-linear calculations using multiple parameters derived from the above-mentioned sensors.