From http://en.wikipedia.org/wiki/Root_mean_square

"Thus the peak value of the mains voltage in the USA is about 120 × √2, or about 170 volts."

Working backwards from that, 1/√2*Vpk=Vavg or 1/1.414*170=120. Assuming a more or less pure sinewave, current follows the same pattern,

However, for anything other than a pure resistive load, voltage and current will be out of phase and you may or may not have a sinewave. You have to take many instantaneous measurements of both voltage and current and average them over time to get true power.