Freight class is a measure of cubic volume used in conjunction with weight to identify how much of a semi-trailer will be needed to accommodate a shipment. Freight class and weight are required to obtain an LTL freight rate estimate.
50 is the lowest freight class and is reserved for the most dense commodities. 500 is the highest freight class and is reserved for the least dense commodities. 200 pounds of a commodity with a freight class of 50 will be significantly smaller in size than one with with a freight class of 500 weighing the same amount. The price charged per pound of freight will increase as the freight class increases since commodities with higher freight classes will consume more of the semi-trailer, leaving less space available for other shipments.
A shipment's freight class is identified by the shipper during the shipment preparation process. Carriers routinely audit the freight class and will correct it if they believe it is incorrect, a practice referred to as reclassing. Reclassing a shipment can have a significant impact on the associated charges, and therefore it is very important to make sure that the freight class of your products is accurately identified.
The official resource for identifying a commodity's freight class is the National Motor Freight Classification (NMFC) directory maintained by the National Motor Freight Transportation Agency (NMFTA.org). You can purchase a hard copy of the directory or obtain an online subscription to it from their web site.
Other resources for discovering the freight classes of your products include:
Doing an online search for freight class calculator will return many options. Use the calculator offered by the carrier you intend to use if one is available. Using a calculator from another carrier or resource may return a different result than what your carrier will use for billing purposes if the commodity is on the threshold of two freight classes (i.e. 65 vs 70). Examine your freight invoices when received to confirm the freight class you are using. Using a freight class that is inconsistent with your carrier's determination will result in a difference between the quoted and billed rate.
Companies tend to sell products that can be grouped into just a few freight classes.