A method for instantaneously determining rates of distortion on variable frequency signals, and an associated device, in which a harmonic distortion rate is calculated across the shortest possible temporal window, corresponding to the duration of a period of a given signal's fundamental. Hence, the aim being to precisely determine the signal frequency value, whose HDR is to be calculated, an iteration of certain measurements, achieved at the time of a given calculation, is embodied in order to calculate the HDR on subsequent signals.