REW's Total Harmonic Distortion (THD) calculation method

[erubio00]

I've read about a few methods to calculate THD based of IEC or IEEE. Is REW calculating THD relative to the fundamental only or fundamental + harmonics?

 

[John Mulcahy]

THD is calculated relative to the fundamental, per the help. THD+N is calculated from the ratio of the input power minus the fundamental power to the total input power (note that it is possible for THD+N to be lower than THD using these definitions).