Dimensional weight is based upon cubic size of package rather than weight. Dimensional weight takes into account the size of package including height, length and depth. These are all used to calculate shipping cost. You may have noticed when you have a package which is 12 x 12 x 8 inches it costs one amount but if the size is 12 x 16 x 16 inches it can be significantly higher cost, even though the box weighs the same. That is because of dimensional weight. Your package will be charged the higher of the dimensional weight, or actual weight.