Class follows pounds per cubic foot.
Cubic feet are length times width times height in inches, divided by 1,728. Density is weight divided by those cubic feet. The bands used here are the common planning steps: under 1 pcf is 400, then 300, 250, 175, 125, 100, 92.5, 85, 70, 65, 60, 55, and 50 once density reaches 50 pcf. A 48 by 40 by 48 inch skid at 500 pounds is 53.33 cubic feet and 9.38 pounds per cubic foot, which lands in class 100. Measure the freight as it will ship, including the pallet, not the bare carton.