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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, and as a unit of volume. 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 barrel of the largest size. This could contain a volume between 210 and 256 gallons (800 to 1000 L), which could weigh around 2,000 pounds (900 kg) and occupy some 60 cubic feet (1.7 m³) of space.^[1]

In the United Kingdom the ton is defined as 2,240 pounds (1,016 kg) (avoirdupois pounds).^[2]. From 1965 the UK embarked upon a programme of metrication and gradually introduced metric units, including the tonne (metric ton), a non-SI metric unit defined as 1000 kg. The UK Weights and Measures Act 1985 explicitly excluded from use for trade many units and terms, including the ton and the term "metric ton" for "tonne"^[3].

In the United States and Canada^[4] 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" is pronounced (/ˈtʌnɪ/)^[5], or "metric ton" when it is necessary to make the distinction.

Where accuracy 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 10% of both. The ton is the heaviest unit of weight referred to 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 2.8 m³) in capacity.

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

In Britain ton is used in slang in several contexts to mean "100". As a cost or a speed, a ton is widely understood to be £100 or 100 mph^[6].

Units of mass/weight[link]

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

Others[link]

• The long ton is used for petroleum products such as aviation fuel.
• 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 (2240 pounds, about 1016 kg). This measurement is also used in the U.S. tonnage of naval ships.
• Increasingly, tonnes are being used rather than long tons in measuring the displacement of ships. See tonnage.
• Harbour ton used in South Africa in the 20th century, 2000 pounds or one short ton.

Both the long ton and the short ton are 20 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).^{[citation needed]} 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^[12].

Assay ton (abbreviation 'AT') is not a unit of measurement, but a standard quantity used in assaying ores of precious metals; it is 29 ¹⁄₆ grams (short assay ton) or 32 ²⁄₃ grams (long assay ton), the amount which bears the same ratio to a milligram as a short or long ton bears to a troy ounce. In other words, the number of milligrams of a particular metal found in a sample of this size gives the number of troy ounces contained in a short or long 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.

Units of volume[link]

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.^[13] 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)^[14]. 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 informal definitions. It is sometimes abbreviated as "MTON".^{[15][16][17][18]} 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"^{[citation needed]} 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 still cause it to be confused with the tonne, or even 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.^[19]

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)^[20], the volume occupied by 1 long ton (2,240 lb; 1,016 kg) of water under the conditions that define the imperial gallon.

Units of energy and power[link]

Ton of TNT[link]

• 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 metric 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 dietary calorie (Cal) is distinct and 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).^[21][22] 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.

Ton of oil equivalent[link]

A ton of oil equivalent (TOE) is a conventional value, based on the amount of energy released by burning one tonne of crude oil, of 41.868 GJ, 11.63 MWh, 1.28 TCE, 39.68 million BTU, or 6.6 - 8.0 actual barrels of oil (depending on actual specific gravity). ^[23]

Ton of coal equivalent[link]

A ton of coal equivalent or tonne of coal equivalent (TCE) is a conventional value of 7 Gcal (IT) = 29.3076 GJ.

Refrigeration[link]

The unit ton is used in refrigeration and air conditioning to measure heat absorption. Prior to the introduction of mechanical refrigeration, cooling was accomplished by delivering ice. Installing one ton of refrigeration 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)."^[24] 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 ton of ice per day.
• A less common usage is the power required to cool 1 long ton (2,240 lb or 1,016 kg) of water by 1 °F (0.556 °C) every 10 minutes = 13,440 BTU/h ≈ 3939 W.^[25][26]

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 °C) which can show more favorable data, which are not valid when making performance comparisons among units unless conversion rates are applied.^{[citation needed]}

The refrigeration ton is commonly abbreviated as RT.

Informal tons[link]

• Ton is also used informally, often as slang, to mean a large amount of something (material or not), for example, "Man, I just ate a ton of french fries back there".
• 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 pretty much anything else.^[27][28]

See also[link]

References[link]