All hair lengths are measured from the top of the crown of the wig/sheitel down to the bottom of the length of the hair. Please see the picture below for measurements. 

The longest layer of the wig will determine the wig length. For example if the longest layer of the wig is 20" long, and the shorter layer is 18" long, the wig will be considered a 20" wig. We recommend ordering a piece with an extra 2" (minimum) to allow for cutting while maintaining your desired length.