Volumetric weight is used in all courier industries to assess the overall size of the parcel in volumetric kilograms. This is calculated by length x widthx height (cm) /5000. Some carriers uses a different divisor ranging from 4000-6000. At times you may find that your parcel is priced by using the volumetric weight rather than the physical weight of the parcel but it is based on whichever is the greatest.
If you have a parcel that is pretty small but weighs 8kg and the parcel dimension are 25cm x 30 cm x 15cm then the volumetric weight would be at 2.25kg. The volumetric weight is lower than means that the pricing would be based on the actual physical weight of 8kg because that is greater of the 2.