What is Chargeable Weight (Volumetric Weight)?
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 of the shipment to ascertain which is greater; the higher weight is used to calculate the shipment cost. 

Calculation
 Multiply the length x height x width of your parcel in centimetres.
 Divide the answer by 5,000.
 The result is the volumetric weight. This figure should
be compared with the actual weight in kilogrammes.
Whichever is the greater figure will be used to calculate
your consignment price.
 Please note that you need to round all weights up to
the next half kilogram.
For example
Let's say the actual weight of your parcel is 10 kg, and dimensions are
30cm x
50cm x
50cm.
 30cm x 50cm x 50cm = 75000
 75000 / 5000 = 15kg
The volumetric weight is therefore 15kg, which is greater
than the actual weight of 10kg, and so the volumetric weight
would be used in calculating the price to be charged.
Please follow this link to find out maximum dimensions per parcel.