This video introduces the concept of “root mean square,” commonly called RMS. This is a mathematical method for determining the average of a constantly varying value. In electronics, RMS is used to calculate effective AC power in a way that allows it to be compared to the equivalent heating value of a DC system. This video begins by defining RMS and explaining why it is needed and how it is used. We then introduce the concept of “averaging” RMS, a technique that is useful when the sinusoidal AC wave is relatively symmetrical and undistorted. We conclude with a discussion of “true” RMS, a more complex mathematical method for calculating effective voltage and current when the AC waveform is non-sinusoidal and/or significantly distorted by factors such as harmonics.