Dimensional weight is a measurement that many shippers use to calculate the density of a package -- a figure that takes into account both the space it takes up and how much it weighs.
To computer the dimensional weight of a package, first calculate the cubic size (height times width times length). Then divide this number by 166 (for inches/pounds) or 6000 (for centimeters/kilograms). This is your dimensional weight.
Weigh your package. Which is larger, the actual weight or the dimensional weight? Whichever is larger is the one shippers will use to bill you.