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 weight. Its original use as a measurement of volume has continued in the capacity of cargo ships and in terms such as the freight ton. Recent specialized uses include the ton as a measure of energy and for truck classification. It is also a colloquial term, "ton" (any definition) is the heaviest unit of weight typically used in colloquial speech. It is also used informally to mean a large amount of something, material or not.
In the United States and Canada, a ton is defined to be 2,000 pounds (907.18474 kg).
Where confusion is possible, the 2240 lb ton is called "long ton" and the 2000 lb ton "short ton". The 1000 kg 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 , 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%.
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, or in refrigeration as a unit of power, sometimes called a ton of refrigeration.
The ton 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.
Units of mass/weight
There are several similar units of mass or volume called the ton:
^ abIn the UK, the words "ton" and "tonne" are usually pronounced the same way, . As the imperial ton and the (metric) tonne only differ by 2%, ambiguity is not typically a problem; where accuracy is required in speech, "long ton" or exaggerated pronunciation of "tonne" emphasising the "e", , are used.
^ abThe longweight and shortweight tons were used as a means of making an allowance for wastage in an industrial process. The workman is provided with a longweight ton and is expected to return a shortweight ton of processed product. These measures were particularly used in the operation of hammering iron blooms into shape.
^In other industries, a different longweight ton might be used. Coal miners delivered coal to the surface in longweight tons but were paid only for a shortweight ton. This was supposedly to allow for "dirt" (non-coal rocks) in the output. Mine owners, however, were free to set the value of the longweight ton at a value of their own choosing, and in at least some cases, it was set to compared to the shortweight ton. This was a source of discontent amongst the miners who saw the practice as unfair in favour of the mine owners.
Other units of mass/weight
Deadweight ton (abbreviation 'DWT' or 'dwt') is a measure of a ship's carrying capacity, including bunker oil, fresh water, ballast water, crew and provisions. It is expressed in tonnes (1000 kg) or long tons (2,240 pounds (1,016 kg)). This measurement is also used in the U.S. tonnage of naval ships.
Harbour ton, used in South Africa in the 20th century, was equivalent to 2000 pounds or one short 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 2240 lb.
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 plantstHM 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.
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.
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). 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 used to determine the amount of money to be charged in loading, unloading, or 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.
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.
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), the volume occupied by 1 long ton (2,240 lb; 1,016 kg) of water under the conditions that define the imperialgallon.
Units of energy and power
Ton of TNT
A ton of TNT or tonne of TNT is a unit of energy equal to 109 (thermochemical) calories, also known as a gigacalorie (Gcal), equal to 4.184 gigajoules (GJ).
A kiloton of TNT or kilotonne of TNT is a unit of energy equal to 1012 calories, also known as a teracalorie (Tcal), equal to 4.184 terajoules (TJ).
A megaton of TNT (1,000,000 tonnes) or megatonne of TNT is a unit of energy equal to 1015 calories, also known (infrequently) as a petacalorie (Pcal), equal to 4.184 petajoules (PJ).
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). The definition applies for both spellings: ton of TNT and tonne of TNT.
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).
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.
In North America, a standard ton of refrigeration is 12,000 BTU/h (3,517 W). "The heat absorption per day is approximately the heat of fusion of 1 ton of ice at 32 °F (0 °C)." This is approximately the power required to melt one short ton (2,000 lb or 907 kg) of ice at 0 °C (32 °F) in 24 hours, thus representing the delivery of 1 short ton (0.893 long tons; 0.907 t) of ice per day.
A less common usage is the power required to cool 1 long ton (2,240 lb or 1,016 kg = 1 long ton or 1.120 short tons or 1.016 t) of water by 1 °F (0.56 °C) every 10 minutes = 13,440 BTU/h (3,939 W).
The refrigeration ton is commonly abbreviated as RT.
Ton is also used informally, often as slang, to mean a large amount of something, material or not. For example, "I have a ton of homework to do this weekend."
In Britain, a ton is colloquially used to refer to 100 of a given unit. Ton can thus refer to a speed of 100 miles per hour, and is prefixed by an indefinite article, e.g. "Lee was doing a ton down the motorway"; to money e.g. "How much did you pay for that?" "A ton" (£100); to 100 points in a game e.g. "Eric just threw a ton in our darts game" (in some games, e.g. cricket, more commonly called a century); or to a hundred of any other countable figure.
In Dutch, when talking about money a ton is used to indicate 100,000. For example a house costing 2 ton would cost 200,000 euros. This convention has been in use since at least the 18th century.
In Finnish, tonni is often used as a synonym for 1000, especially when referring to money. For example, tonnin seteli was a 1000-markbanknote, and a popular TV show was called Kymppitonni ("ten tons" = 10,000 marks).
^"Naval Architecture for All". United States Bureau of Transportation Statistics. Archived from the original on October 10, 2008. Retrieved 2008.. "Historically, a very important and standard cargo for European sailing vessels was wine, stored and shipped in casks called tuns. These tuns of wine, because of their uniform size and their universal demand, became a standard by which a ship's capacity could be measured. A tun of wine weighed approximately 2,240 pounds, and occupied nearly 60 cubic feet." (Gillmer, Thomas (1975). Modern Ship Design. United States Naval Institute.) "Today the ship designers standard of weight is the long ton which is equal to 2,240 pounds." This is the weight of 35 cubic feet of Sea Water at a specific gravity of 1.025, compared to Fresh Water, specific gravity of 1.000 usually measured at 60 degrees F. Handy numbers: 35, 36, 37, number of Cubic Feet per Salt Water, Fresh Water and Lube Oil.
^Chris Evans, Göran Rydén, Baltic iron in the Atlantic world in the eighteenth century, p.257, Brill 2007 ISBN90-04-16153-8
^"Report of the select committee on mines", Reports from Committees 1866, vol.9, pp.134-136, London: House of Commons, 23 July 1866
^Definitions of 2000, 2240, 2352, and 2400 lb are included in citations listed in the Oxford English Dictionary, 2nd ed. OED cites an 1858 dictionary of trade products "the legal ton by weight is usually 20 cwt".