What is the RMS value of AC sinusoidal waveform?

General formula: square root of the square modulus averaged over a period:

xRMS =1/T sqrt( integral (|x(t)|2dt) ) ,

where x(t) is the signal and T is its period.

If you solve it for sinusoidal waves, you get a 1/sqrt(2)~0.707 factor between peak amplitude and RMS value:

xRMS ~ 0.707 XPK ~ 0.354 XPK-PK ~ ...