Loaded displacement tonnage is the actual weight of the ship and cargo.
Light displacement tonnage is the actual weight of the ship.
DWT Deadweight Tonnage: The difference between the loaded displacement and the light displacement is the weight that the ship can actually carry and is known as the ‘deadweight’ tonnage.
DWT= Loaded DT - Light DT
Size of tankers is usually expressed in deadweight tonnage, eg a 250,000 ton dwt tanker means it can carry 250,000 tons of oil (CARGO), bunkers and stores at its summer draft. It is more convenient when transporting liquids to charge for the ton weight carried, not only because it is a relatively heavy cargo, but the volume of 250,000 tons of oil can appreciably change with, say, a 10-degree variation in temperature.