Deadweight tonnage (DWT) — how much a ship can carry

Deadweight tonnage is the total weight a ship can carry when loaded to her legal limit — cargo, fuel, fresh water, ballast, stores, crew and passengers all counted together, in tonnes. Crucially it does not include the weight of the ship herself. It is the number a shipowner cares about most, because it is the ship's carrying capacity and therefore her earning power.

The definition is beautifully simple: deadweight equals the ship's loaded displacement minus her light displacement. Load the ship until her Plimsoll line touches the water, weigh the whole thing (by reading her draft and consulting her hydrostatic tables), then subtract the weight of the empty ship. What's left is everything she picked up — the deadweight.

This is why the giants of this site's tanker and bulk-carrier boards are ranked by DWT and not by volume. Seawise Giant's 564,763 tonnes deadweight is the most weight any ship has ever been built to lift. Container ships, by contrast, run out of space before they run out of weight capacity, so they are ranked by TEU instead — deadweight is the wrong ruler for a box ship.