When is a ton not a tonne?

An explanation of short and long tons

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Shutterstock / Brian Clifford / A circular road sign with a white background and a red border. The sign shows a small semi-truck (lorry) in black with the weight 7.5 t (for long ton) embossed in white across it. Green foliage is visible behind the sign.

The units of measure known as “ton” and “tonne” can cause confusion for authors and editors alike. Both are units of mass; however, they do not describe the same unit of mass. Let’s take a look at each unit, and learn a bit of their history, as this will help to distinguish between the two.

Standardisation of units

Over the centuries, there have been many attempts to standardise units of measure. In 12th century England, for example, Henry I declared that the standard yard should be the distance from the tip of his nose to the end of his thumb. Not long after, the British Parliament declared a standard inch to be the length of three grains of barley laid end to end. In fact, ancient Roman measurements of distance were based on the average foot length of a Roman Centurion. So we can see that attempts at standardisation usually had their basis in a measure that was anything but standard.

To add confusion, where standard measures from the English (imperial) system were disseminated to other countries, the foreign governments had the power to change them. This partly explains why a ton in England isn’t the same weight as a ton in the United States.

In the 1700s, the French Academy of Sciences was assigned the task of creating a new standardised system of measures. The result, after many years, was the metric system (also known as the International System of Units, or SI). While some countries do not exclusively use the metric system (such as the United States), all countries have recognised and adopted the SI.

Long ton

A long ton is a unit in the imperial system, and it is generally used in the United Kingdom and Commonwealth countries. A long ton is equal to 2,240 pounds (1,106 kg).

Short ton

A short ton is generally used in the United States. Although it is derived from the imperial unit, a short ton is equal to 2,000 pounds (907.2 kilograms).

Both the long ton and the short ton have a common definition: 1 ton = 20 hundredweight. However, the definition of hundredweight differs depending on the country. In the United Kingdom, there are 112 pounds to a hundredweight, and in the United States, there are 100 pounds. This is why, even though the long ton and short ton have the same definition, their actual weights differ between the countries.

Tonne (or metric ton)

A tonne is a unit of mass in the metric system. It is defined in the SI system as 10³ kg (1,000 kg or 2,204.6 pounds). It is also referred to as a metric ton, to differentiate it from the long and short ton in non-metric use.

Shipping ton

Unlike the units of mass, the ton and the tonne, described above, a shipping ton is a measure of volume. The actual weight depends on the commodity being shipped, and the unit is equivalent to 40 cubic feet (1.1 m³) in the United States and 42 cubic feet (1.2 m³) in the United Kingdom.

A number of other units in shipping, and other industries, use the word “ton”. These include a displacement ton (35 cubic feet of seawater or 2,240 pounds), a freight ton (40 cubic feet or 1,000 kg) and a register ton (100 cubic feet, relating to the internal capacity of a ship).

In summary

Knowing where the term ton/tonne is being used can help you clarify which unit of measure it relates to. In the United States, the author is probably referring to a short ton (2,000 pounds). Elsewhere, the author could be referring to a metric tonne (1,000 kg) or a long ton (2,240 pounds). If in doubt, query with the author or source to ensure the correct unit is used.