A ventilator includes a compressor, a storage vessel, a valve assembly communicating with the compressor, the storage vessel, and an inhalation line. A controller directs the valve assembly between a storage configuration where ventilation gas is delivered from the compressor into the storage vessel, and a delivery configuration where ventilation gas is delivered from the storage vessel to the inhalation line. The controller is coupled to a pressure sensor for detecting a first pressure within the storage vessel when the valve assembly is directed to the delivery configuration, and detecting subsequent pressure while ventilation gas is delivered from the storage vessel to the inhalation line, the controller determining the volume of ventilation gas delivered to the patient based at least in part on the difference between the first pressure and the subsequent pressure.