Dimensional weight is a formula used to calculate the volume weight of a shipment and it is based on the actual dimensions of the piece. (I.e. a carton that's dimensions are 42" x 30" x 36" and only weighs 120 lbs, would dim out to 260 dimensional pounds. The same carton would "dim out" to 274 dimensional pounds internationally.) Dimension weight takes up "space" while actual weight is the exact weight of the piece. Here are the formulas. Each shipment is based on actual or dimensional weight, whichever is greater.
(Length x Width x Height / dim factor of 175 = The Dimensional weight Domestically)
(Length x Width x Height / dim factor of 166 = The Dimensional weight Internationally)