Both the long ton and the short ton are composed of twenty hundredweights, each having different values for the hundredweight (112 and 100 pounds respectively). Prior to the 15th century in England, the ton was composed of 20 hundredweights, each of 108 lb, giving a ton of 2,160 pounds.
Assay ton (abbreviation 'AT') is not a unit of measurement (nobody ever has x assay tons of something), but rather a standard quantity used in assaying ores of precious metals; it is grams (short assay ton) or 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; however, in more recent documents the spelling tonne refers exclusively to the metric ton.
In the context of nuclear power plants, tHM and MTHM mean (metric) tonnes of heavy metals, and MTU means metric tonnes of uranium. In the steel industry, the acronym THM has the meaning '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 ton is a unit of volume used for describing the displacement of (usually) a warship. It represents the volume of water displaced by the ship. One measurement ton is equal to 35 cubic feet or 0.99108963072+ cubic metres. Coincidentally, 35 cubic feet of water weighs very close to 1 tonne, but the tonne had not been defined when the displacement ton was. Consequently it is less approximately equal to 1 long ton of water (2240 lbs). It is usually abbreviated as DT.
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 today equal to 40 cubic feet of space (1.132 cubic metres), but historically it has had several informal definitions. It is correctly abbreviated as 'FT' but some users are now using freight ton to represent a weight of 1 tonne, thus the more common abbreviations are now M/T, MT, or MTON (for measurement ton), which still cause it to be confused with the metric ton or even the megaton.
The register ton is also a unit of volume used for the cargo capacity of a ship, defined as 100 cubic feet (roughly 2.83 cubic metres). 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 or about 1.88 cubic yards.
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 was formerly used in Great Britain and equal to 224 imperial gallons (the volume occupied by a mass of one long ton under the conditions which define the imperial gallon).
(Note that volume tons are units of convenience used in shipping and are not useful in science except that they are exactly defined.)
Note that these are small calories (cal). The dietary Calorie (Cal) is distinct and equal to one kilocalorie.
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, 4.184 kilojoules) 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.
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.
The refrigeration ton is abbreviated as RT providing scope for confusion with the register ton.
A ton is also equivalent to 100 runs in the game of cricket (more commonly known as a century).