The handling requirements, liability, the ability to stow the freight and the density in terms of height, weight and length determine the freight class of a shipment. Using these four characteristics of the shipment, the National Motor Freight Classification can be determined by using special software, such as ClassIT or Fast Class, or by using the NMFC booklet available through The National Motor Freight Traffic Association website.
The density of a shipment is calculated by dividing the weight of the shipment in pounds by the volume in cubic feet. To determine the volume in cubic feet, multiply the height of the freight by the length and width in feet. If measuring in inches, convert to cubic feet by dividing the final product by 1,728. Calculators exist online that determine the density for the user after the user puts in the necessary data, such as the height and weight of the shipment. The lower the density, the higher the freight class the shipment receives. For example, freight with a 1 pound per cubic foot density may receive a classification of 400, whereas freight with a 40 pound per cubic foot density may receive a classification of 55. Overall, there are 18 different freight classes.
The cost of shipping increases with a decrease in density and increase in weight class. How the freight is packaged and delivered also affects the freight class. For example, if the product can't be stacked, loaded with other material, or it if contains hazardous materials or special transport requirements, this increases the freight class. Shipments that contain perishable cargo or that have a high probability of damage or theft may require a classification based on value per pound instead of density, though density is still an important factor.