The ton is a unit of measure. It has a long history and has acquired a number of meanings and uses over the years. It is used principally as a unit of mass. Its original use as a measurement of volume has continued in the capacity of cargo ships and in terms such as the freight ton. It can also be used as a measure of energy, for truck classification, or as a colloquial term.

It is derived from the tun, the term applied to a cask of the largest capacity. This could contain a volume between 175 and 213 imperial gallons (210 and 256 US gal; 800 and 970 l), which could weigh around 2,000 pounds (910 kg) and occupy some 60 cubic feet (1.7 m3) of space.^[5]

In the United Kingdom the (Imperial) ton was formerly a unit of Statute measure, defined as 2,240 lb (1,016 kg)^[1][6] From 1965 the UK embarked upon a programme of metrication and gradually introduced metric units, including the tonne (metric ton), defined as 1,000 kg (2,205 lb). The UK Weights and Measures Act 1985 explicitly excluded many units and terms from "use for trade", including the ton (and the term "metric ton" for "tonne").^[7]

In the United States and Canada^[8] a ton is defined to be 2,000 pounds (907 kg).

Where confusion is possible, the 2240 lb ton is called "long ton" and the 2000 lb ton "short ton"; the tonne is distinguished by its spelling, but usually pronounced the same as ton, hence the US term "metric ton". In the UK the final "e" of "tonne" can also be pronounced (/ˈtʌni/),^[9] or "metric ton" when it is necessary to make the distinction.

Where precision is required the correct term must be used, but for many purposes this is not necessary: the metric and long tons differ by only 1.6%, and the short ton is within 11% of both. The ton (any definition) is the heaviest unit of weight typically used in colloquial speech.

The term "ton" is also used to refer to a number of units of volume, ranging from 35 to 100 cubic feet (0.99 to 2.83 m3) in capacity.

It can also be used as a unit of energy, expressed as an equivalent of coal burnt or TNT detonated.

In refrigeration, a ton is a unit of power, sometimes called a ton of refrigeration. It is the power required to melt or freeze one short ton of ice per day. The refrigeration ton hour is a unit of energy, the energy required to melt or freeze ​1⁄24 short ton of ice.

There are several similar units of mass or volume called the ton:

Both the long ton and the short ton are 20 hundredweight, the long hundredweight and the short hundredweight being 112 and 100 pounds respectively. Before the twentieth century there were several definitions. Prior to the 15th century in England, the ton was 20 hundredweight, each of 108 lb, giving a ton of 2,160 pounds (980 kg). In the nineteenth century in different parts of Britain, definitions of 2240, 2352, and 2400 lb were used, with 2000 lb for explosives; the legal ton was usually [sic] 2240 lb.^[15]

Assay ton (abbreviation 'AT') is not a unit of measurement but a standard quantity used in assaying ores of precious metals. A short assay ton is ​29 1⁄6 grams while a long assay ton is ​32 2⁄3 gram. These amounts bear the same ratio to a milligram as a short or long ton bears to a troy ounce. Therefore, the number of milligrams of a particular metal found in a sample weighing one assay ton gives the number of troy ounces of metal contained in a ton of ore.

In documents that predate 1960 the word ton is sometimes spelled tonne, but in more recent documents tonne refers exclusively to the metric ton.

In nuclear power plants tHM and MTHM mean tonnes of heavy metals, and MTU means tonnes of uranium. In the steel industry, the abbreviation THM means 'tons/tonnes hot metal', which refers to the amount of liquid iron or steel that is produced, particularly in the context of blast furnace production or specific consumption.

A dry ton or dry tonne has the same mass value, but the material (sludge, slurries, compost, and similar mixtures in which solid material is soaked with or suspended in water) has been dried to a relatively low, consistent moisture level (dry weight). If the material is in its natural, wet state, it is called a wet ton or wet tonne.

In the Imperial units system, the imperial (long) ton is equivalent to 20 hundredweight, a hundredweight being eight stone, and a stone weighing 14 pounds.

The displacement, essentially the weight, of a ship is traditionally expressed in long tons. To simplify measurement it is determined by measuring the volume, rather than weight, of water displaced, and calculating the weight from the volume and density.^[16] For practical purposes the displacement ton (DT) is a unit of volume, 35 cubic feet (0.9911 m3), the approximate volume occupied by one ton of seawater (the actual volume varies with salinity and temperature).^[17] It 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 is equal to 40 cubic feet (1.133 m3), but historically it has had several different definitions. It is sometimes abbreviated as "MTON". It is used to determine the amount of money to be charged as "Freight" in carrying different sorts of cargo. In general if a cargo is heavier than salt water, the actual tonnage is used. If it is lighter than salt water, e.g. feathers, freight is calculated using Measurement Tons of 40 cubic feet.^{[18][19][20][21]} The freight ton represents the volume of a truck, train or other freight carrier. In the past it has been used for a cargo ship but the register ton is now preferred. It is correctly abbreviated as "FT" but some users are now using freight ton to represent a weight of 1 tonne (1,000 kg; 2,205 lb), thus the more common abbreviations are now M/T, MT, or MTON (for measurement ton), which may cause it to be confused with the megatonne.

The register ton is a unit of volume used for the cargo capacity of a ship, defined as 100 cubic feet (2.832 m3). It is often abbreviated RT or GRT for gross registered ton. (The former providing confusion with the refrigeration ton). It is known as a tonneau de mer in Belgium, but, in France, a tonneau de mer is 1.44 cubic metres (50.85 cu ft).

The Panama Canal/Universal Measurement System (PC/UMS) is based on net tonnage, modified for Panama Canal billing purposes. PC/UMS is based on a mathematical formula to calculate a vessel's total volume; a PC/UMS net ton is equivalent to 100 cubic feet of capacity.^[22]

The water ton is used chiefly in Great Britain, in statistics dealing with petroleum products, and is defined as 224 imperial gallons (35.96 cu ft; 1.018 m3),^[23] the volume occupied by 1 long ton (2,240 lb; 1,016 kg) of water under the conditions that define the imperial gallon.

Note that these are small calories (cal). The large or dietary calorie (Cal) is equal to one kilocalorie (kcal), and is gradually being replaced by the latter correct term.

Early values for the explosive energy released by trinitrotoluene (TNT) ranged from 900 to 1100 calories per gram. In order to standardise the use of the term TNT as a unit of energy, an arbitrary value was assigned based on 1000 calories (1 kcal or 4.184 kJ) per gram. Thus there is no longer a direct connection to the chemical TNT itself. It is now merely a unit of energy that happens to be expressed using words normally associated with mass (e.g., kilogram, tonne, pound).^[24][25] The definition applies for both spellings: ton of TNT and tonne of TNT.

Measurements in tons of TNT have been used primarily to express nuclear weapon yields, though they have also been used since in seismology as well.

A tonne of oil equivalent (toe), sometimes ton of oil equivalent, is a conventional value, based on the amount of energy released by burning one tonne of crude oil. The unit is used, for example, by the International Energy Agency (IEA), for the reported world energy consumption as TPES in millions of toe (Mtoe).^[26]

Other sources convert 1 toe into 1.28 tonne of coal equivalent (tce).^[28] 1 toe is also standardized as 7.33 barrel of oil equivalent (boe).^[29]

A tonne of coal equivalent (tce), sometimes ton of coal equivalent, is a conventional value, based on the amount of energy released by burning one tonne of coal. Plural name is tonnes of coal equivalent.

The unit ton is used in refrigeration and air conditioning to measure the rate of heat absorption. Prior to the introduction of mechanical refrigeration, cooling was accomplished by delivering ice. Installing one ton of mechanical refrigeration capacity replaced the daily delivery of one ton of ice.

A refrigeration ton should be regarded as power produced by a chiller when operating in standard AHRI conditions, which are typically 44 °F (7 °C) for chilled water unit, and 95 °F (35 °C) air entering the condenser. This is commonly referred to as "true ton". Manufacturers can also provide tables for chillers operating at other chilled water temperature conditions (as 65 °F or 18.3 °C) which can show more favorable data, which are not valid when making performance comparisons among units unless conversion rates are applied.

The refrigeration ton is commonly abbreviated as RT.