Shipping Costs, for the most part ... are calculated by weight. However, if your goods are bulky, or can`t be packed into a standard dispatch box, your order will likely require more than 1 box, or a large, oversize box.
Goods like packs of Crisps, that have lots air inside are typical examples. Large boxes of Chocolates are another, where once packed the overall box is oversized.
In these cases, your parcel will be calculated "volumetrically" during checkout, and the price will be more than the standard delivery. This is necessary as the volumetric "weight" of the package is determined by the amount of space it occupies in transit. (So rather than a large weight, it a large bulky parcel)
If the volumetric weight exceeds the packed weight of the package then UKmail charges the greater weight.
The 2013 Volumetric rate is 40p per kg. The standard conversion formula is: Length (cms) x width (cms) x height (cms)/4000 as shown in the example diagram below.