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Weight-based systems
Displacement Tonnage is the actual weight of the vessel and its contents. It is often used to rate naval vessels, since their weight is fairly constant and they are not subject to the kinds of port fees that are calculated on Register Tonnage. It is often expressed in long tons or in metric tons, and is calculated simply by multiplying the volume of the hull below the waterline (ie. the volume of water it is displacing) by the density of the water. (Note that the density will depend on whether the vessel is in fresh or salt water, or is in the tropics, where water is warmer and hence less dense.) For example, in sea water, multiply the volume in cubic feet by 64 (the weight of one cubic foot of seawater) to get the weight of the ship in pounds; or divide by 35 to calculate the weight in long tons.

The word "displacement" arises from the basic physical law, discovered by Archimedes, that the weight of a floating object equates exactly to that of the water which would otherwise occupy the "hole in the water" displaced by the ship.

Light ship tonnage measures the actual weight of the ship with no fuel, passengers, cargo, water, etc. on board.

Deadweight Tonnage is the maximum weight that a ship can safely carry when fully loaded. It includes the crew, passengers, cargo, fuel, water, and stores. Like Displacement Tonnage, it is often expressed in long tons or in metric tons.[5]

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Origins
Historically, Tonnage and Poundage was the tax on tuns (casks) of wine shipped to England, mostly from Spain and Portugal, under a subsidy granted to the English crown by Parliament starting in the 14th century.