Freight class and freight density are closely connected, but they are not exactly the same thing. Understanding this difference is one of the first steps toward using a Freight Class calculator correctly. Freight density describes how much a shipment weighs compared with the amount of space it occupies. It is normally expressed in pounds per cubic foot. A shipment that is heavy but takes up relatively little space has a high density. A shipment that weighs less while taking up a large amount of space has a lower density.