Typically, large items with a light overall weight can be charged according to the space they take up on aircraft.  In these cases, Volumetric Weight, or dimensional (Dim) weight, is used to calculate the shipment cost. It is recommended that you calculate the Volumetric Weight for every shipment that you send, then compare this to its actual weight. The greater weight of the two is used to work out the price that the freight company will charge you.

International Volumetric Weights are calculated using the formula below:

(Length x Width x Height in centimeters) / 6000 = Volumetric Weight in kilograms

The following definition is from Wikipedia: Volumetric weight, also known as Dimensional weight is used to used by postal companies and other freight industries and courier services around the world to invoice for the actual space that a parcel, package or pallet takes up in a vehicle or storage area. A vehicle can quickly become full of bulky, lightweight articles long before it reaches it's capacity in weight which makes for inefficient use of space. The basic procedure for calculating volumetric weight is: find the smallest cuboid box that your consignment will fit in. find the volume of this notional box in cubic centimetres. divide this by 6000: the answer is your volumetric weight in kilograms. The result of this calculation is that if your consignment's density is one-sixth that of water or more, then its volumetric weight is less than the normal weight in kilograms, whereas if its density is less than one-sixth that of water, then its volumetric weight is more than its weight in kilograms. Transport and postal companies will charge based on the greater of these two calculations. Above Definition from Wikipedia February 24, 2009.  The most current definition can be found at https://en.wikipedia.org/wiki/volumetric_weight