Abstract: Indicating fit status of a mask in communication with a respiratory assistance device is disclosed. Upon initiating a therapeutic gas delivery from the respiratory assistance device to the mask, one or more measurements from respective one or more sensors of the respiratory assistance device is received. A leakage value from these measurements is derived, and a mask fit index is assigned. This is based at least upon a correlation of the leakage value to a particular sub-range of predetermined leakage values that corresponds to the mask fit index. The particular sub-range of predetermined leakage values is one among a plurality of sub-ranges, which together comprises an overall mask fit range defined at least by an ideal mask fit region, a loose mask fit region, and a tight mask fit region. A mask fit status based upon the assigned mask fit index is output to an indicator interface.