The functional residual capacity (FRC) value of a subject's lungs determined for each breath is related a volume dilution factor kn and the final FRC value is determined at the point where the volume dilution factor kn reaches a predetermined volume dilution factor value. The determination of the final FRC value can be based on interpolation or extrapolation from a measured breath data series. The volume dilution factor may be normalized value. This is achieved by division of all volume dilution factors with the initially determined volume dilution factor. With normalization, the compartments in which gas concentration change is achieved slowly are referenced with the faster, more effective compartments, thus giving a ventilation profile of the lungs.