Dimensional Weight is a standardize calculation used by all the big three shippers to access if your package is taking up an undo amount of space. For an extreme example, imagine that you shipped one pen in a box made for a computer. The weight would practically be negligible but it would take up a lot of space in the shipping truck that could be used to ship heavier items. The shipping companies seek compensation for this in the form of the dim weight designation. Normally you do not have to worry about this unless you are shipping a non-heavy item in a large box.

US DOMESTIC AIR SERVICE
(Length x Width x Height = Cubic Inches)/(194)

US DOMESTIC GROUND SERVICE
(Length x Width x Height = Cubic Inches)/(194)

INTERNATIONAL SERVICE
(Length x Width x Height = Cubic Inches)/(166)

Good Luck and Good Shipping,
Your How To Ship Advisor