Calculation of peak value for ac signal?

The peak value of a sinusoidal ac signal is RMS voltage divided by 0.707, or two times the square root of two.

Note: AC signals are rarely truly sinusoidal, because power supplies rarely pull a consistent load for the full duty cycle. (They pulse.) This introduces harmonics in the signal, which cause error in the above calculation but, unless the harmonics are extreme, the calculation is close.