Almost all U.S. LTL carriers use NMFC freight classes, although some are attempting to move to a dimensionally-based system.

The National Motor Freight and Traffic Association has 18 freight classes numbered 50 to 500. The lower the freight class, the lower the cost of transporting that freight.

Some items, such as car transmissions, have a specific class no matter how heavy or big they are. This is called a fixed class. Items can be assigned a fixed class if the value and transportability very rarely change.