This disclosure describes systems and methods for compensating for leakage when determining the respiratory mechanics of a patient during delivery of gas from a medical ventilator to a patient. The systems and methods include monitoring an instantaneous flow in the ventilation system based on one or more measurements of pressure and flow in ventilation system. The leakage is modeled using a predetermined leakage model. A leak-compensated instantaneous lung flow of gas inhaled or exhaled by the patient is estimated. Using the leak-compensated lung flow and a predetermined respiratory mechanics model, the respiratory mechanics of a patient may be estimated including at least one of a leak-compensated lung compliance and a leak-compensated lung resistance. The method may include using a dynamic respiratory mechanics model or a static respiratory mechanics model that requires use of a pause maneuver to collect the necessary data.