The accurate measurement of RMS values of voltage and current is crucial for the monitoring and protection of power systems and in general for electrical power distribution systems. The authors in this paper have developed an algorithm to calculate the RMS value of sinusoidal signals of varying frequency, by applying phasors obtained from digital filters SAL and CAL. In conditions in which the frequency of the grid varies, a phase shift is presented in the phasor of the grid voltage which is used for the design of a regulator that allows to obtain accurate RMS voltage values. The choice of the SAL and CAL digital filters is due to their low computational requirement, so they can be implemented in general-purpose microcontrollers.