A method of estimating leak flow in a gas delivery system is provided that includes determining an average total flow Qtot (or summation or integral of total flow) of the gas delivery system for each of N breaths, determining Pγ1 and Pγ2 for each of the N breaths, wherein P is a leak pressure of the gas delivery system, γ1 is a first predetermined value, γ2 is a second predetermined value, and Pγ1 and Pγ2 are averages for the breath (summation or integrals may also be used), setting up a system of N equations, one for each of the N breaths, wherein each of the equations has the form Qtot=Gorf.Pγ1+Gvg.Pγ2 and uses the determined Qtot, Pγ1 and Pγ2 for the associated breath, solving the system of N equations for Gorf and Gvg, and using Gorf and Gvg to calculate instantaneous leak Qleak using Qleak=Gorf.Pγ1+Gvg.Pγ2