Ton

A ton is a unit used to measure mass, volume, or force, with a history that has given it several distinct meanings. Originally a volume unit, it still appears in cargo ship capacity and measures like the freight ton.

As a unit of mass, a ton can refer to the long ton (2,240 pounds or 1,016.0 kilograms), the metric ton or tonne (1,000 kilograms, about 2,204.6 pounds), or the short ton (2,000 pounds or 907.2 kilograms). The short ton differs from the other two by about 10%, while the metric and long tons differ by less than 2%. The metric tonne is often spelled with an "e" to distinguish it, though in the U.S. and U.K. it is pronounced the same as "ton" and usually called a "metric ton" when clarity is needed. In the U.K., the final "e" of "tonne" may also be pronounced; in Australia, it has its own pronunciation. In Ireland and most Commonwealth nations, a ton is 2,240 pounds (1,016.04691 kg). In the U.S. and Canada, a ton is 2,000 pounds (907.18474 kg). Before the 20th century, definitions varied: in England before the 15th century, a ton was 20 hundredweight of 108 pounds each, totaling 2,160 pounds (980 kg). In 19th-century Britain, definitions of 2,240, 2,352, or 2,400 pounds were used, with 2,000 pounds for explosives; the legal ton was usually 2,240 pounds. The UK long ton and US short ton both equal 20 hundredweight, but the long hundredweight is 112 pounds (51 kg) and the short hundredweight is 100 pounds (45 kg). In the U.K., Canada, Australia, and other former imperial-system areas, the tonne is the legal trade ton.

Other mass units include the deadweight ton (DWT or dwt), which measures a ship's carrying capacity—including bunker oil, fresh water, ballast, crew, and provisions—expressed in tonnes or long tons. This is also used for U.S. naval ship tonnage. Increasingly, tonnes replace long tons for ship displacement. The harbour ton, used in 20th-century South Africa, equaled 2,000 pounds (907 kg), or one short ton. The assay ton (AT) is not a measurement but a standard quantity for assaying precious metal ores: a short assay ton is about 29.17 grams (1.029 oz), and a long assay ton about 32.67 grams (1.152 oz). These ratios match those of a short or long ton to a troy ounce, so milligrams of metal in a sample weighing one assay ton indicate troy ounces per ton of ore. In pre-1960 documents, "ton" was sometimes spelled "tonne," but later "tonne" refers only to the metric ton. In nuclear power, tHM and MTHM mean tonnes of heavy metals, and MTU means tonnes of uranium. In steel, THM means "tons/tonnes hot metal," referring to liquid iron or steel produced. A dry ton or dry tonne has the same mass but refers to material (sludge, slurries, compost) dried to low moisture; in its natural wet state, it is called a wet ton or wet tonne.

Volume units include the displacement ton (DT), a unit of 35 cubic feet (0.9911 m³)—roughly the volume of one ton of seawater—used for ship displacement. This is slightly less than the 224 imperial gallons (1.018 m³) of the water ton (based on distilled water). One measurement ton or freight ton equals 40 cubic feet (1.133 m³), though it has had several definitions historically. It determines charges for loading, unloading, or carrying cargo: if cargo is heavier than salt water, actual weight is used; if lighter (e.g., feathers), freight is calculated in measurement tons of 40 cubic feet. Gross tonnage and net tonnage are volumetric measures of a ship's cargo capacity. The Panama Canal/Universal Measurement System (PC/UMS) uses net tonnage modified for billing, with a PC/UMS net ton equal to 100 cubic feet of capacity.

Because the ton is usually the heaviest unit in everyday speech, its name is also used figuratively to mean a large amount or degree, as in "There's a ton of bees in this hive," "We have tons of homework," or "I love you a ton." The word "ton" comes from "tun," a term for the largest cask, which held between 175 and 213 imperial gallons (210 to 256 US gallons; 800 to 970 liters), weighed around 2,000 pounds (910 kg), and took up about 60 cubic feet (1.7 m³) of cargo space.