Measuring the capacity of a ship is derived from the following methods:
- Displacement tonnage. The total weight of a ship when fully loaded to the waterline (or baseline), which is the line of maximal safe loading. It is measured by using the corresponding weight of the water being displaced by the volume of the ship under the waterline in metric tons.
- Lightweight tonnage. The total weight of a ship when empty and measured by using the weight of the water being displaced in metric tons. It includes the hull and all the equipment (engine) but not the bunker and stores.
- Deadweight tonnage. The maximum weight that a ship can carry (its carrying capacity), which represents the displacement tonnage minus the lightweight tonnage. It includes bunker, ballast, and stores.
- Gross tonnage. The total volume of a ship converted into tonnage and measured as the volume of all enclosed spaces expressed in cubic meters. This figure is often used to calculate harbor fees and tolls for canal transits through the class that a ship’s gross tonnage belongs to.
- Net tonnage. The volume available for cargo transport obtained by subtracting the volume occupied by the engine room and the space necessary for the ship’s operation (ballast, crew quarters, bridge) from the gross tonnage.
Containerships have a unique capacity measure related to the number of twenty-foot equivalent unit boxes (TEU) they can carry without exceeding the waterline or impairing visibility from the bridge. Containers can be carried above and below the main deck, so a containership can have its TEU capacity expressed accordingly. Two figures are commonly used:
- Slot capacity. The total number of containers (TEU) that can be put in the carried slots if they have little or no cargo. This capacity feature is fixed and volumetric.
- Loadable capacity. The total number of containers (TEU) that can be loaded on a ship, based on weight and stability limits. Loadable capacity is lower than slot capacity unless the containers are empty. This capacity feature varies depending on the cargo weight, its distribution within the cargo holds, and trade balance (repositioning empties).