A gas delivery system for delivery of breathable gas to a patient comprising: a pressure generator that pressurizes the breathable gas for delivery to the patient; a pressure sensor that measures a pressure difference between the pressurized breathable gas and atmospheric pressure; a flow sensor adapted to measure a standard flow rate of the pressurized breathable gas output by the pressure generator; and wherein the gas delivery system further comprises: a processor that calculates a density correction factor based at least in part on the pressure difference and the measured standard flow rate and multiplies the standard flow rate of the pressurized breathable gas by the density correction factor to determine a volumetric flow rate of the pressurized breathable gas, characterized in that the gas delivery system further comprises a valve downstream from the pressure generator, the valve being controlled by the processor; wherein the density correction factor is C = T m T std P std P m, wherein T m represents a measured absolute temperature, T std represents standard temperature, P std represents standard pressure, and P m represents a measured absolute pressure of the pressurized breathable gas, and wherein the processor is configured to control the valve to bring the flow rate of the pressurized breathable gas to substantially zero while the pressure generator is operating, and, when the flow rate is substantially zero, the processor calculates the density correction factor based at least in part on an assumption that the standard flow rate is zero. a pressure generator that pressurizes the breathable gas for delivery to the patient; a pressure sensor that measures a pressure difference between the pressurized breathable gas and atmospheric pressure; a flow sensor adapted to measure a standard flow rate of the pressurized breathable gas output by the pressure generator; and wherein the gas delivery system further comprises: a processor that calculates a density correction factor based at least in part on the pressure difference and the measured standard flow rate and multiplies the standard flow rate of the pressurized breathable gas by the density correction factor to determine a volumetric flow rate of the pressurized breathable gas, characterized in that the gas delivery system further comprises a valve downstream from the pressure generator, the valve being controlled by the processor; wherein the density correction factor is C = T m T std P std P m, wherein T m represents a measured absolute temperature, T std represents standard temperature, P std represents standard pressure, and P m represents a measured absolute pressure of the pressurized breathable gas, and wherein the processor is configured to control the valve to bring the flow rate of the pressurized breathable gas to substantially zero while the pressure generator is operating, and, when the flow rate is substantially zero, the processor calculates the density correction factor based at least in part on an assumption that the standard flow rate is zero. a processor that calculates a density correction factor based at least in part on the pressure difference and the measured standard flow rate and multiplies the standard flow rate of the pressurized breathable gas by the density correction factor to determine a volumetric flow rate of the pressurized breathable gas, characterized in that the gas delivery system further comprises a valve downstream from the pressure generator, the valve being controlled by the processor; wherein the density correction factor is C = T m T std P std P m, wherein T m represents a measured absolute temperature, T std represents standard temperature, P std represents standard pressure, and P m represents a measured absolute pressure of the pressurized breathable gas, and wherein the processor is configured to control the valve to bring the flow rate of the pressurized breathable gas to substantially zero while the pressure generator is operating, and, when the flow rate is substantially zero, the processor calculates the density correction factor based at least in part on an assumption that the standard flow rate is zero.