If its a sine wave youre measuring, mulitply by 2*sqrt(2), or 2.8284. For RMS to Vpeak its RMS*sqrt(2) but since you want peak-peak, its 2x that.
If its not a sine wave, then make a voltage doubler,
(the transformer in the image is the AC source, substitute with whatever you want to measure) and add ~0.6 to 1V for the voltage dropped over the diodes. The drop would nominally be 1.4V for the 2 diodes, but with no load being drawn, then the Vf of the diodes will drop a lot.