Volumetric weight is a measure of the space a parcel occupies in the delivery vehicle. Even if your surfboard only weighs 10kg, it occupies the same space on the van as a large number of smaller parcels, therefore the size of your parcel is influencing the cost of delivery.
If your parcel is not heavy, but it is very long or irregularly-shaped, then some services will compute the delivery cost using the volumetric weight of the item.
These services are:
Parcelforce International: (Length (cm) * Weight (cm) * Length (cm)) / 5000
UPS/DHL - all services: (Length (cm) * Weight (cm) * Length (cm)) / 5000
Redhead: (Length (m) * Weight (m) * Length (m)) * 333
For advice on converting between measurement units, please click here.