Almost all U.S. LTL carriers use NMFC freight classes, although some are attempting to move to a dimensionally-based system.
Adding NMFC code to a shipment is technically optional, but the NMFC codes can help avoid reclassification and ensure you're using the correct freight class number.
Multiply the length, width, and height of your shipment, then divide the total weight of your package by that number. If your shipment is 4 feet long, 5 feet wide, and 4 feet tall, you would multiply 4 x 5 x 4 to get 80 cubic feet. If it weighs 800 pounds, you would divide 800 / 80 to get 10 pounds per cubic foot.