Two weighing methodologies are permitted:
Method 1: Upon the conclusion of packing and sealing a container, the shipper may weigh, or arrange a third party to weigh, the packed container.
Method 2: the shipper or, by arrangement of the shipper, a third party may weigh all packages and cargo items, including the mass of pallets, dunnage and other packing and securing material to be packed in the container, and add the tare mass of the container to the sum of the single mass of the container’s contents.
More information and explanations are available on the WSC (World Shipping Council) website.