Tonnage is a measurement used primarily in shipping to describe either the internal volume or the carrying capacity of a vessel. Despite the name, tonnage doesn’t always refer to weight. In maritime contexts, some forms of tonnage measure enclosed space inside a ship, while others measure how much weight a ship can carry. The term also appears in air conditioning, where it describes cooling power. Which meaning applies depends entirely on the context.
Why Tonnage Doesn’t Always Mean Weight
The word “tonnage” traces back to medieval taxes on wine barrels called “tuns,” and that origin helps explain the confusion: from the start, tonnage was about capacity, not weight. In modern shipping, some tonnage figures describe volume (how big a ship is inside), while others describe mass (how heavy a ship or its cargo is). Mixing them up leads to wildly misleading comparisons, so the industry maintains distinct terms for each.
Gross Tonnage and Net Tonnage
Gross tonnage (GT) is a nonlinear measure of a ship’s overall internal volume. It accounts for the total enclosed space of the vessel, including crew quarters, engine rooms, storage areas, and every other internal space. Gross tonnage is not expressed in pounds or kilograms. It’s a dimensionless number derived from a mathematical formula applied to the ship’s total enclosed volume.
Net tonnage (NT) is calculated the same way but covers only cargo spaces. It strips out crew quarters, machinery rooms, navigation areas, and other non-revenue spaces. By rule, a ship’s net tonnage can never fall below 30% of its gross tonnage.
Both figures were standardized by the International Convention on Tonnage Measurement of Ships, adopted by the International Maritime Organization in 1969 and enforced starting July 18, 1982. Before that, countries used variations of a system invented by George Moorsom of the British Board of Trade in 1854, which led to significant inconsistencies. The convention replaced the older “gross register tons” and “net register tons” with the current GT and NT system, creating a single universal standard for the first time.
These numbers matter because port authorities, canal operators, and governments use them to calculate fees and regulations. The Suez Canal, for instance, requires ships to report both their Suez Canal Net Tonnage and Suez Canal Gross Tonnage to estimate transit tolls. In the United States, Customs and Border Protection charges a tonnage tax on vessels arriving from foreign ports based on net tonnage, at rates of either 2 cents or 6 cents per net ton depending on where the ship originated.
Displacement Tonnage
Displacement tonnage is the one form of tonnage that actually measures weight. It’s based on Archimedes’ principle: the weight of a floating ship equals the weight of the water it pushes aside. This can be calculated by measuring the volume of water displaced and converting it to mass.
There are two key versions. Light displacement (sometimes called lightweight tonnage or LDT) is the weight of the empty ship itself, with no cargo, fuel, fresh water, ballast, stores, passengers, or crew aboard. Loaded displacement is the weight of the ship plus everything on board for a voyage: cargo, passengers, fuel, water, provisions, and supplies. The difference between those two numbers is essentially what the ship is carrying.
Deadweight Tonnage
Deadweight tonnage (DWT) measures the total weight a ship can safely carry. This includes cargo, fuel oil, diesel, lubricating oil, fresh water, ballast water, provisions, crew, and their personal effects. It’s the gap between a ship’s light displacement and its maximum loaded displacement.
When you see a cargo ship described as “80,000 DWT,” that number represents the upper limit of everything the vessel can take on board before it sits too low in the water. DWT is the figure most commonly used to compare the carrying capacity of commercial vessels, particularly bulk carriers and tankers. It’s worth noting that DWT includes fuel and water alongside cargo, so the actual amount of revenue cargo a ship can haul is always somewhat less than its full deadweight figure.
How These Measurements Are Used
Each type of tonnage serves a different practical purpose:
- Gross tonnage determines manning requirements, safety regulations, registration fees, and port dues. A larger GT means more crew, more safety equipment, and higher fees.
- Net tonnage is used for canal transit tolls and government tonnage taxes. Since it reflects cargo space, it ties costs to a ship’s commercial potential.
- Displacement tonnage is most relevant in naval architecture and shipbuilding, where engineers need to know the actual physical weight of the vessel.
- Deadweight tonnage is the standard for commercial freight, helping shippers and charterers compare how much different vessels can transport.
Tonnage in Air Conditioning
Outside of shipping, “tonnage” most commonly appears in heating and cooling. A ton of refrigeration is a unit of cooling power, defined as the energy needed to melt or freeze 2,000 pounds (one short ton) of ice in 24 hours. That works out to 12,000 BTU per hour, or roughly 3.5 kilowatts.
When a residential AC unit is described as a “3-ton system,” it can remove 36,000 BTU of heat per hour from your home. The term has nothing to do with the weight of the equipment. Most homes use systems ranging from 1.5 to 5 tons, with the right size depending on square footage, insulation, climate, and other factors. An oversized unit cycles on and off too frequently, while an undersized one runs constantly without keeping up.
Quick Reference
- Gross tonnage (GT): Ship’s total internal volume. Not a weight. Unitless number.
- Net tonnage (NT): Volume of cargo spaces only. Always at least 30% of GT.
- Displacement tonnage: Actual weight of the ship, measured in long tons or metric tons.
- Deadweight tonnage (DWT): Maximum weight a ship can carry, in metric tons.
- Refrigeration ton: 12,000 BTU/hour of cooling power.