Quantifying the effect of voltage fluctuations in power systems using a flickermeter is documented in IEC 61000-4-15. Modern digital implementations of a flickermeter take continuous time voltage samples as inputs to a software processing algorithm which ultimately calculates the short-term flicker severity P st . While this process is perfectly suited for real-time measurement applications, it is less useful for off-line applications typically encountered during planning studies or any other application where continuous time sample data are not available. Because rms values are typically produced as a result of planning calculations and cycle-be-cycle rms values are much easier to store over long time periods for later use in off-line evaluations, it is appropriate to consider the performance of the flickermeter algorithm with rms voltage inputs so that additional applications can be considered. The analytical evaluation of flickermeter performance with rms voltage inputs is presented in this paper. The conclusion is that the flickermeter algorithm performs accurately with these inputs provided that the fluctuation frequency is low, but becomes inaccurate for higher frequency fluctuations. This analytical conclusion is supported by both simulation and field test results at three sites.