What is the volumetric weight 

No matter you use any freight methods to send a 100 kg cotton or 100 kg papers to oversea countries, or same logistics needs in domestic,  no doubt the cubic meter of a 100 kg cotton is bigger than a 100 kg paper is. the volumetric weight of a shipment is a calculation that reflects the density of a package, a less dense item generally occupies more volume of space, in comparison  to its actual weight, The volumetric or dimensional weight is calculated and compared with the actual weight for each piece in the shipment to ascertain which is greater; the higher weight is used to calculate the total shipment cost.  so normally, the chargeable measure for a shipment cost is to whichever bigger,

How to calculate volumetric weight 

by FCL sea shipping; FCL shipment applies a container’s max weight loading

by LCL sea shipping; LCL shipment normally 1 cbm = 500 kg, some countries different 

by air freight;  (length x width x heigh) ÷ 6000 unit is centimeter 

by courier express ; (length x width x heigh) ÷ 5000 unit is centimeter