Sometimes termed deadweight carrying capacity, is the difference between the light and loaded displacements of a ship or barge. The Deadweight tonnage comprises the cargo, stores, ballast, fresh water, fuel oil, passengers, crew and their effects. Gross tonnage is a measure of the internal capacity of a ship, tug or barge. It is a cubic or space measurement of all areas of a vessel with some allowances or deductions for exempt spaces such as living quarters. Net tonnage is a measure of the internal capacity of a ship, tug or barge's cargo space volume only.
Lightweight tonnage is the weight of the ship itself.
The hull, machinery and all its fittings. Fuel, stores and crew are not included. Samuel Plimsoll, a British merchant and shipping reformer campaigned for this mark to prevent ships from being overloaded and lost at sea. His idea became law in Britain in 1875. The mark, a circle with a horizontal line through the center was painted on both sides of a ship.