Almost all U.S. LTL carriers use NMFC freight classes, although some are attempting to move to a dimensionally-based system.
The first step in determining your freight class is to take measurements. Measure the height, width and depth of your shipment then multiple those three measurements together for the total cubic feet. Then divide the total cubic inches by 1,728 (the number of cubic inches in a cubic foot). Finally, divide the weight (in pounds) of the shipment by the total cubic feet.
If you don't include an NMFC code on your BOL, it is very likely that your carrier will charge you a reclassification fee and charge you the cost of shipping the freight they determine your shipment is.