In mathematics, the root mean square (abbreviated RMS or rms), also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids. It can be calculated for a series of discrete values or for a continuously varying function. The name comes from the fact that it is the square root of the mean of the squares of the values. - Wikipedia
General Equation for RMS Voltage:
RMS (effective) value of any periodic function with period T.
Average of Instantaneous Power:
RMS Equation for a Sine Wave:
, ...for positive or negative half-cycle