In common parlance and everyday use, when we use word 'weight', we generally mean 'mass', and the SI unit for mass is kilogram. However, in science and technology, weight means force which is measured in newton. In addition to kilogram, there is another unit which we often use in commerce to measure mass; this unit is 'metric ton'. It is represented by the symbol 't' and is equal to 1,000 kilograms (i.e. 1 t = 103 kg). In many countries (e.g. U.K.), this unit is called 'tonne'. However, in the US, the term 'metric ton' is used. Metric ton is sometimes abbreviated as 'T' or 'MT'; both of these symbols represent unapproved usage.
In 1960, the eleventh General Conference on Weights and Measures (CGPM or Conférence Générale des Poids et Mesures) established the International System of Units. This system is abbreviated as SI, after the French title Le Système International d’Unités. This system is based on seven base quantities (length, mass, time, electric current, temperature, amount of substance and luminous intensity) and their units (meter, kilogram, second, ampere, kelvin, mole, candela). In addition to the seven base units, there are derived units which are expressed algebraically in terms of the seven base units or other derived units. For example, derived quantity, speed’s unit is 'meter per second' and its symbol is m/s.
There are many important units which are widely used but are not part of the SI, they are outside the SI. These outside or non-SI units could be placed into two groups. The first group has units which are accepted for use with the SI by the Comité International des Poids et Mesures (CIPM). The second group comprises units which are not accepted for use with the SI by the CIPM. Metric ton is one of the non SI units in the first group.
Ton and Pounds
Ton is an old term which has been in use for a long time. This term is not included in the SI system. A ton is defined as equal to 20 hundredweights. However, the definition of hundredweight is different in the US and the UK. One hundredweight is equal to 112 pounds in the UK and it is equal to 100 pounds in the US. Therefore, in the UK, a ton is equal to 20 x 112 = 2,240 pounds and in the US it is equal to 20 x 100 = 2,000 pounds. To avoid confusion, the two tons are called as Long Ton and Short Ton. A long ton is equal to 2,240 pounds and a short ton corresponds to 2,000 pounds.
There are many other usages of the word ton. For example, Assay Ton (AT) is a standard quantity of ore taken for assaying (testing). This quantity is equal to 291662⁄3 milligrams. In the English language, the word ton is used for representing a great quantity.