A metric ton is a unit of measurement based on the metric system, rather than the standard, or imperial, system more commonly used in the United States. It is known as a tonne in most other countries, and is equivalent to 1,000 kilograms. Strictly speaking, it is a megagram, being one million grams.
In countries that employ the metric system, tonne is the correct term for this unit of measure, although it is often referred to as a ton. In the United States, however, “ton” usually means another, non-metric, unit known as the “short ton” — equivalent to 2,000 pounds — or, less commonly, a “long ton” — 2,240 pounds — which is the same as the imperial ton formerly used in the UK. In the USA, this measure is still used in the freight industry and for navy ships. Due to the similarity in pronunciation — and, in some cases, spelling — there can often be confusion between short, long and metric tons. The terms “long” and “short” come from units used in the UK before conversion to the metric system and are largely obsolete outside the USA.
Due to the differing systems and terminology used by the USA and other countries, and within the USA for different applications, it is useful to be able to convert one unit to another. The main conversions, and synonyms, are summarized below:
Measurement Systems and SI Units
The measurement system employed in the USA, known as United States customary units, is very similar to the imperial system employed in the UK prior to the adoption of metric units. There are, however, some differences: these arise from the fact that the system is based on UK units that were in use before the imperial system was standardized. The short ton was adopted in the USA prior to UK imperial standardization, after which the imperial unit became the slightly heavier long ton. The tonne was sometimes called a tonneau or millier — both French terms — but these are no longer used.
It has been recommended that the USA should switch fully to the metric system. This, however, has met with considerable resistance from commerce and consumers. As a result, metric measurements are normal only for scientific, medical and military purposes, and the short ton is still the standard for common use.
The official symbol for the metric ton in the International System of Units (SI) is simply “t.” Although the unit itself is not an SI unit, it is accepted for use with the SI system. All of the standard prefixes can be used with it, but in practice the most common ones are kilotonne and megatonne.
Measuring Energy and Force
The units kilotonne and megatonne may be used for measuring energy release. In this context, energy is related to the amount released by detonation of the equivalent mass of trinitrotoluene (TNT), an explosive used in bombs during World War II. These units are most commonly used to measure the explosive power of nuclear weapons. A metric ton of TNT releases roughly 4.19 x 109 joules of energy, with a kilotonne releasing 4.19 x 1012, and a megatonne releasing 4.19 x 1015. To put this in some context, the Hiroshima bomb was equal to about twenty kilotonnes of TNT, while a modern nuclear missile could fall closer to twenty megatonnes.
The metric ton is sometimes used as a unit of force. Strictly speaking, it should be called the tonne-force, but is often simply called the tonne, or, in the USA, metric ton. For example, it might be stated that a bulldozer can exert a force of ten metric tons. The SI unit for force, however, is the newton, and the tonne or tonne-force is not accepted as part of the SI system.
The word most likely comes from the Latin term tunna, which is the word for a cask. Since a large cask filled, for example, with wine, would weigh roughly a metric ton, this origin is commonly accepted. The word has been in use for quite some time, although previously the spelling was more often tunne.