Volumetric weight, is a pricing technique used within commercial freight transport, which uses an estimated weight that is calculated from the length, width and height of a package(s).
The shipping fee is then based upon the dimensional weight or the actual weight, whichever is greater.
Reason being, if only charged on weight, lightweight, low density packages become unprofitable for freight carriers due to the amount of space they take up in the lorry/aircraft/ship in proportion to their actual weight.
Volumetric weight is either calculated by multiplying the length, width and height of a parcel (in cm) and dividing that figure by 4000 for road services & 5000 for express air & UK domestic services.
It may also be calculated by its girth, with the following calculation: 1 x length + 2 x width + 2 x height.
Providing the correct dimensions are declared at the point of booking, the RDS portal will calculate and quote, based on the greater of the two measurements.