This tool converts from deciBel (dB) to Percentage Distortion.
Percent Distortion to dB
ALinear = 10AdB/20
A% = 100*ALinear
Harmonic distortion (sometimes called Total Harmonic Distortion or THD for short) is the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency  in an output waveform.
In any electronic system, whether it is audio, radio frequency or power, the lower the THD the better.