Vpeak to peak is defined as the voltage measured between the maximum Positive and Negative Amplitudes on the sine wave. 
The Vrms is defined as square root of the mean of the squares of the values for the one time period of the sine wave.
Following is the formula for Vpp to Vrms conversion.
Vrms = 0.3535 * Vpp
Where Vpp is the peak to peak volatge and Vrms is the root mean square voltage. To Calculate Vrms from Vpp enter the value in Vppand hit Vrms button.