In the present work the authors present the realization of a virtual instrument implementing a self-calibrating THD meter. The instrument is able to draw an electrical signal provided from a phase of the electrical distribution network, and to evaluate its total harmonic distortion (THD). Actually the instrument calibration process is developed in two times: at first a "black box calibration" is executed (it concerns the system consisting, for a quite general purpose digital device, in: voltage/current transducers + ADCs + Data Processing including the FFT calculation and the determination of the amplitude of the fundamental frequency component); the process is performed by using a reference instrument every time it is consider necessary (that's to say every time the period of use might be cause of not too good performances of the instrument). The results, opportunely stored as a matrix of partial accuracies in the non-volatile memory of the data processing unit, allow the determination of the uncertainty related to the THD calculation through an algorithm in conformance with The ISO Guide on the expression of the uncertainty of indirect measurements (this last represents the second step of the calibration procedure). For this reason the whole calibration act is defined by the authors of the present paper as a "grey box calibration". During the measurement, in fact, the instrument gets ready to the self-calibration by using a fuzzy representation of the harmonic components, so to achieve the expression of the cited power quality index in terms of an interval defined for a specified confidence level selected by the user. The proposed methodology proves to be of fast and simple application not only for virtual instruments, but also for stand-alone measurers based on DSP or FPGA technologies