CBM stands for "cubic meter" in shipping. This measurement is calculated by multiplying the width, height and length together of one's carton, and if one has multiple cartons to ship, by adding the CBMs of each carton together.
The width, height and length must be in meters before the calculation is performed. One inch equals 2.54 centimeters, and there are 100 centimeters in a meter. So, if one has a 20-inch by 20-inch by 20-inch box, each side would be 50.8 centimeters, or .508 meters. Thus the CBM of the box would be .13 cubic meters. If one had 100 of these boxes to ship, one's total CBM volume for the shipment would be 13.