http://www.allaboutcircuits.com/vol_2/chpt_1/3.html

You have the form factor listed incorrectly twice once in the text, once in the review/summary section as being the ratio of the crest (peak) to the average value.

It is actually the ratio of the RMS value to the average value. In in the case of a sine wave the form factor is supposed to be 1.11 = .707/.637.

While I don't have a specific reference giving this equation handy. The following IEEE paper, mentions on pg 207 (page 4 of the paper) that the form factor of a sine wave is 1.11.

*R.E. Morrison, Parameters that influence power quality on industrial frequency AC traction systems, Proceedings of the IEEE Power Engineering Society Transmission and Distribution Conference, vol. 1, pp. 204-209, Summer 2001.*

While I was looking up how a form factor was calculated I found several sites mentioning that it was the ratio of RMS to average, which does give the expected value of 1.11. While looking around for a more offical site I came across yours which appears to be in error. Using your calculation gives an incorrect value of ~1.57 for the form factor of a sine wave waveform.