The crest factor of a waveform is given by the ratio of the peak to RMS amplitude level
- Peak amplitude
- Root-mean-square Amplitude
CF = Vpeak/VRMS
If the peak value is 1V and RMS value is 0.707, then the Crest factor is 1.414 or 3 dB (using the volt to dB calculator). These are the values for a sine wave .
The crest factor for an OFDM waveform is quite a bit higher at 4 or 12 dB.
 Crest factor on Wikipedia